Mankind learned early on in our history that in order to protect a piece of land you needed to fortify it. From humble beginnings, the concept would evolve to produce some of the world's Great Fortresses of all time.
The very word fortification, by extension fortress or fort, is derived from Latin fortis ("strong") and facere ("do" or "to make"). To "make strong" would be a concept that would drive humans to build ever more complex and impressive defensive structures.
Starting out as simple earthworks and wooden walls, fortifications would evolve into the highly complex and imposing Citadels of the Middle Ages. Gunpowder and canons would render these monumental structures obsolete and force a change to low rise earthworks, e.g. star forts, commonly seen from the 18th century onwards.
The development of explosive shells once again revolutionized fortress design. They would now need to be made of concrete and steel and be partially buried, leading to the bunkers common throughout the 19th and 20th centuries.
By the advent of WW1, static fortifications had become largely redundant and, in most cases, a move towards demilitarized zones became more popular.
In the following article, we will look at 21 fantastic examples of the world's greatest fortresses. This list is not exhaustive and is no particular order.
1. Rock of Gibraltar - As strategically important today as in antiquity
First on our list of great fortresses is one that would be of great strategic importance for millennia.
The Rock of Gibraltar, or simply 'The Rock', is a monolithic limestone promontory in the British overseas territory of Gibraltar. Today, a large proportion of the upper area of the Rock is a nature reserve and is famed for its Barbary macaques.
In antiquity, it was called one of the Pillars of Hercules but the Romans called it Mons Calpe. The other Pillar of Hercules, Mons Abyla or Jebel Musa, is located on the African side of the Straits of Gibraltar.
These two points once marked the limit of the known world.
'The Rock' is the site of an old Moorish Castle which stands as a relic of their former 700-year rule of Gibraltar and Spain in general. The castle was built around 711 AD.
Gibraltar was famously ceded to the United Kingdom in 1704 during the war of the Spanish Succession.
Spain would try and fail to retake the territory for many years to come. It is still, today, a place of major strategic importance for Britain.
2. Rumeli, Istanbul - Built to Take on the Romans
Rumeli Castle, Rumelihisari or Boğazkesen Hisarı in Turkish, is a medieval fortress built by the Ottomans in a gambit to capture Constantinople. Its Turkish name literally means "Strait-Cutter Castle" a name it would ultimately live up to.
It was the brainchild of Ottoman Sultan Mehmed II who built it between 1451 and 1452. Its construction was, in part, a preparation for a siege of Constantinople.
The idea was to prevent easy military and logistical relief to the city using the Bosphorus Strait. Rumeli and her sister fort, Anadolu Hisari (Anatolian Fortress) on the opposite bank would, ultimately, turn the tide of the siege.
Starved of logistical support from the Bosphorus, Constantinople would later fall in 1453. It would later serve as a customs checkpoint and prison.
Today it's an open-air museum and well worth a visit.
3. Dover Castle - England's Largest Castle
Dover Castle in Dover, Kent, England, is a medieval castle still as formidable today as it was hundreds of years ago.
It was constructed in the 11th century and is colloquially known as 'The Key to England'. It has, unsurprisingly, had a significant defensive role in the history of Great Britain.
It is believed that Dover Castle has been a fortified site as early as the Iron Age, at least predating the Roman invasion of AD 43. Its most significant period followed the Battle of Hastings in 1066.
After their decisive victory, the new Norman overlords began a campaign of fortifying their new prize. One such fortification would become what we know today as Dover Castle.
Its current form really took shape under Henry II in the 13th century. Further massive rebuilding took place during the height of the Napoleonic Wars.
It was during this time that massive gun batteries were added and significant remodeling works took place. Its military importance continued right through until WW2.
Today it is a monument of enormous national significance and is Grade I listed.
4. Murad-Janjira, India - The fort that was never defeated
Marud-Janira is the name given to an impressive fort on an island of the coastal village of Murud, India. Janjira, as a word, is not native to India and it is likely of Arabic origin. Probably derived from Jazeera (meaning island).
It was built by the Abyssynian minister of the Sultan of Ahmednagar in the 17th Century. It is estimated that during its height the fortress could bring to bear around 572 cannons.
It is, famously, the only fort off the west coast of India that has never been conquered. Murad-Janjira has repulsed attacks from many hostile nations including the Netherlands, Portugal and even the British.
The fort is, in effect, a huge heavily fortified island. It is still relatively intact to this day and comes complete with its own battery of rusting cannons.
Since its early construction in the year 1736, the fort was under the control of the Siddi. The Siddi are an ethnic group of people who inhabit India and Pakistan and are mixed faith society.
After the Independence of India in 1947, the fort's ownership was passed to the new Indian State.
5. Prague Castle - The Czech Baroque Fortress
Prague Castle is a large complex in Prague that dates to around the 9th century. It used to be the seat of power of the Bohemian Kings, Holy Roman Emperors and later the presidents of Czechoslovakia.
Today it still houses and protects the Bohemian Crown Jewels in a hidden, locked and secret chamber.
The castle occupies a piece of land roughly 570 meters long by 130 meters wide. Apart from the impressive fortification itself, the castle contains many other impressive pieces of architecture.
The castle's skyline is dominated by Saint Vitus Cathedral, for example, a magnificent Gothic cathedral.
Today it is one of the most popular tourist attractions in Prague receiving an estimated 1.8 million visitors every year.
6. The Tower of London - Once feared now loved
Her Majesty's Royal Palace and Fortress of the Tower of London, or the Tower of London for short, is an example of the world's great fortresses that needs no introduction.
It can trace its origins to the year 1066 and the Norman Conquest of England. William the Conqueror ordered its construction in 1078.
As soon as it was built it became a symbol of Norman oppression and a place to be feared. Despite its reputation amongst the populace, it was initially intended as a royal residence.
From 1100, the castle began to be used as a prison, a role it played until as late as the 1950's.
It has been the site of some of the most prominent moments in British History. The Tower has even been besieged a few times in its history.
It has served as an armory, a treasury, a zoo, the home of the Royal Mint, a public record office, and the home of the Crown Jewels of England throughout its life.
Today it is a must-see landmark in London, not to mention the United Kingdom.
7. Citadel of Aleppo - Still fighting today
The Citadel of Aleppo is an imposing example of the world's great fortresses that can be found in the old city of Aleppo in Syria.
It is widely considered to be one of the largest and oldest castles in the world. Archaeological evidence suggests the site has been occupied since at least the 3rd century BC.
The Citadel has, for centuries, been occupied and developed by the Greeks, Byzantines, Ayyubids, and Mamluks. But its existing form is primarily from the Ayyubid period of occupation.
To recognize its historical importance, it was made a UNESCO World Heritage Site in 1986.
The castle itself is surrounded by a deep moat and contains an amphitheater, palace, Turkish baths and underground passages.
In the recent turmoil of Syria, the Citadel was badly damaged during the Battle of Aleppo in 2015. The external gate was shelled in an exchange between the Free Syrian Army and Syrian Army in a class for control of the Citadel.
It re-opened in 2017 and is still undergoing repairs.
8. Bourtange Fort - The star-shaped fortress that turned into a village
Bourtange Fort in the Netherlands was originally built during the Eighty Years' War between 1568 and 1648. It was commissioned and built by William I of Orange who wanted to control the only road between Germany and the Spanish controlled city of Groningen.
Circa 1594, Bourtange was integrated into the general fortification chain on the border of the area's Northern Provinces. The fort experienced its final battle in 1672 and continued to be part key part of the country's defensive network.
The current imposing star fort dates from the 18th century. It would later be abandoned militarily and became a peaceful village in around 1851.
Circa 1960 living conditions in the village deteriorated to such an extent that is was abandoned. It was decided that Bourtange would be rebuilt to its 1740-1750 state to be used as an open-air museum.
9. Caerphilly Castle - Britain's great water defenses
Caerphilly Castle is a formidable medieval example of the world's great fortresses found in Caerphilly, South Wales. It was commissioned and built by Gilbert de Clare in the 13th century as part of his campaign to subdue the locals of the county of Glamorgan.
It would be the site of vicious fighting between Gilbert, his descendants and, local Welsh 'rebels'. The site would become heavily fortified with massive masonry walls and equally impressive earthworks.
Caerphilly Castle is surrounded by a series of enormous artificial lakes which are widely considered the "most elaborate water defenses in all Britain" - according to the historian, Allen Brown.
Occupying a site of around 30 acres, Caerphilly Castle is the second largest in Britain. It is also famed for its introduction of concentric layers of defensive walls to the United Kingdom.
Caerphilly is also well known for its iconic and almost impregnable gatehouse.
Its defenses were not just for show, they were tested repeatedly during the 13th and 14th centuries. Llywelyn ap Gruffudd assaulted the castle in 1271, Madog ap Llywelyn led a revolt against it in 1294, followed by Llewellyn Bren in 1316 and it provided a safehouse during the unrest that followed the overthrow of Edward the II in 1326.
It would fall into disuse during the 15th Century finally becoming a ruin in the 16th century. The Marquesses of Bute acquired it in 1776 who began a massive campaign to restore it to its current condition.
Today it is a popular tourist spot in South Wales and is now owned by the state and managed by CADW.
10. The Ankara Castle that saw rome fall and Turkey rise
Ankara Kalesi (Ankara Castle) is an impressive fortification in the middle of the Republic of Turkey's capital city, Ankara. It was built in antiquity and has changed hands many times throughout the ages.
Ankara Castle comprises a series of concentric defensive walls. The inner walls have closely spaced towers that enclose an area of roughly 350 by 150 meters.
The less well preserved outer walls have less closely spaced towers every 40 meters or so.
It has, over its lifetime, been controlled by Roman and Byzantine forces, but was captured by the Seljuk Turks in 1073. The Crusaders recaptured it in 1101 and the Seljuks would later win it back in 1227.
Under the Ottomans, it would undergo a period of decline and decay but would be partially restored in 1832.
The walls are built from large amounts of masonry a large amount of which is recycled from older buildings in the city. It is not uncommon to spot Roman and Greek tombstones within the walls of the castle.
Today it offers unparalleled views of Turkey's Capital City and is, interestingly, still inhabited today.
11. Spiš Castle - One of Europe's largest castles, covering 41,000 km
Spiš Castle in Slovakia is one of Europe's largest castle sites. It is sited above the town of Spišské Podhradie and the village of Žehra in Slovakia.
By area, the site covers over 41,000 km and is a UNESCO World Heritage site as of 1993.
The castle was founded in the 12th Century and is a combination of Romanesque, Gothic and Renaissance Architecture. Like many of the world's Greatest Fortresses, it was built on the site of a much earlier fortification.
Prior to 1464, the castle was owned by the Kings of Hungary. It would later become the property of various prominent Hungarian families until it was donated to the Czechoslovakian, later Slovakian, state.
The castle was tragically destroyed by fire in 1780. There is some debate whether this was an accident or a deliberate act of vandalism to save the Csaky family from paying taxes.
Whichever is the truth, the castle fell into a period disuse and disrepair. It was partially rebuilt in the latter half of the 20th century.
12. Mehrangarh Fort - One of India's biggest forts, 125 meters above the city
Mehrangarh Fort is one of India's largest forts. It can be found in Jodhpur, Rajasthan. It was built around 1460 by Rao Jodha and looms 125 meters above over the nearby city.
The entire fortress is enclosed by a set of imposing thick wall that protects a series of palaces. These are known for their exquisite carvings and expansive courtyards.
The battlements still show signs of its violent history. It is not uncommon to find impact marks from cannonballs fired at it by the armies of Jaipur.
Mehrangarh has a total of seven gates that were built by Maharaja Man Singh to celebrate his victory over the armies of Jaipur, Udaipur, and Bikaner.
Today the fort houses a variety of museums, exhibitions and craft bazaars. It has also featured in some films, notably the 2012 movie The Dark Knight Rises.
13. Krak des Chevaliers - One of the world's best preserved Crusader castle
Formerly called Crac de L'Ospital, Krak des Chevaliers or Hoṣn al-Akrād, is an old Crusader fortress in Syria. It is one of the most important and well-preserved castles of the age in the world and a fine example of the world's greatest fortresses.
It was built on a former Kurdish settlement inhabited around the 11th century by the Mirdasids. For this reason, it is known locally as the "Castle of the Kurds".
Krak des Chevaliers was gifted to the Knights Hospitaller by Raymond II, Count of Tripoli, in 1142. It would remain under their command until it was besieged and lost in 1271.
The current castle was rebuilt between the 1140's and 1170 by the Knights Hospitaller after it was damaged by an earthquake.
At its height, the castle housed around 2,000 people. During the 1250's the Knights power began to wane and the castle was captured by Mamluk Sultan Baibars in 1271.
During the 1930's the castle was gifted to France who began a program of clearing and restoration of the building. Ownership passed to Syria in 1946 when they won their independence.
It was partially damaged in the Syrian civil war from shelling which, to date, has not been repaired. The full extent of the damage is unknown, but there have been reports of hasty repairs.
15. Conwy Castle - Edward I's Welsh masterpiece
Conwy Castle is one of Britain's biggest and historically important medieval castles. It was built by Edward I during his highly successful conquest of Wales between 1283 and 1289.
It was built as part of some larger works to wall the city of Conwy. According to contemporary accounts, the costs for the combined defenses of the city were around £15,000.
Conwy Castle would play an incredibly vital role in several wars over the centuries following its completion. Like Caerphilly Castle, it withstood a siege from Welsh rebel Madog ap Llewelyn in 1294.
Richard II also used the castle as a safe haven in 1399 and it was held by forces loyal to Owain Glyndwr in 1401.
It would also play a vital role during the English Civil War. Loyalist forces held the castle for many years until their reluctant surrender in 1646. Parliamentary forces then partially demolished the castle to prevent it being recaptured and reused.
Following this, it would fall into disuse and became a ruin in 1665. Like many ruins around the UK, it became a favored destination for artists throughout the 18th and 19th centuries.
Today it is a very popular tourist attraction and is managed by CADW.
16. Fort de Douaumant - The star of Verdun's sheild of iron
Fort de Douaumant, or simply Fort Douaumont, was the highest and largest fort in a ring of 19 similar structures intended to protect the city of Verdun in France and was built in the 1890's.
It consisted, primarily, of a network of underground tunnels levels protected with steel and reinforced concrete 12 meters thick.
The fort was equipped with rotating and retractable turrets, with a mixed armament of 75 mm guns and machine gun turrets.
The French General staff, by 1915, quickly realized that these forts were wholly inadequate to resist fire from German 420 mm Gamma Gun batteries. And so, they ordered them to be partially disarmed and manned them with skeleton crews.
This would prove to be a serious tactical mistake and the fort was taken without loss by a small German raiding party in 1916. An event that would come to epitomize the Battle for Verdun.
The Battle of Verdun would be one of WW1's most bloodthirsty battles, lasting 9 months with a terrible loss of human life.
It would eventually be recaptured by French forces in 1916, officially bringing a close to the battle. Today it is WWI war memorial and museum.
17. Alcázar of Toledo - A Spanish Legend
Once a 3rd Century Roman Palace,Alcázar of Toledo is now stone fort on the highest point in Toledo, Spain.
Once a dilapidated building it was restored by Charles I (also Holy Roman Emperor Charles V) and his son Philip II of Spain in the mid 16th century.
Alcazar is most famed for its part in the Spanish Civil War. Colonel José Moscardó Ituarte managed to resist a siege from an overwhelming Spanish Republican force.
This event would make the fort a central piece of Spanish Nationalist lore in times to come. The building was severely damaged by the end of the siege but it would be rebuilt after the war's conclusion.
It has since been heavily restored and the former fortress is now a museum and a library.
18. Edinburgh Castle - Scotland's safehouse
Edinburgh Castle dominates the modern city's skyline and is one of the UK's greatest fortresses. Like all great fortifications, it is strategically positioned on top of an ancient volcanic plug.
Archaeological studies indicate that the Castle Rock has been occupied since at least the start of the Iron Age. The highly defensible location has been fortified since at least the reign of David I in the 12th century.
It would remain as a royal place of residence until the 17th century when its role changed to that of a military barracks. The castle would be involved in many historical conflicts throughout Scottish history.
It played a prominent role in the Wars of Scottish Independence in the 14th century and it was also heavily involved in the Jacobite uprising of 1745. It is estimated that at least 26 separate sieges have been undertaken against the castle in its 1100 year history.
From the early 19th century its importance as a Scottish symbol has grown from strength to strength. Owing to this, it underwent a series of restoration programmes that still continue to the present day.
Today it is one of Scotland's most visited landmarks. It is also the venue for the annual Edinburgh Military Tattoo and, of course, the Edinburgh International Festival.
19. Malbork Castle - The world's largest brick castle
Malbork Castle, formerly Marienburg, is the world's largest brick-built castle and one of its greatest fortresses. It was built by the Teutonic Order in 1406 after their conquest of Poland sometime before.
The Order was established during the Third Crusade to the Holy land in 1190. At first, it operated as a brotherhood as hospitallers in Acre, then, after receiving its rule in 1198, the brotherhood transformed into a knightly order.
In 1309 the Grand Master of the Order moved his office to Malbork. At this time it was convent house but it would soon morph into the mighty citadel we see today.
This transformation added deep moats and concentric rings of thick defensive walls. The castle was further strengthened during the 14th and 15th centuries with the addition of cannon installations and other outbuildings like a brewery and stable.
Today it is a museum and library and a popular tourist destination in Poland.
20. Königstein Fortress - Europe's largest existing hillfort
Königstein Fortress, otherwise known as the 'Saxon Bastille', is a 13th century hilltop fortress near Dresden, Germany. To this day it still keeps watch over the nearby town of Königstein on the bank if the River Elbe.
It is one of Europe's largest hilltop fortifications and is one of the world's Greatest Fortresses. It is built on 9.5-hectare (95000 m2) site, 240 meters above the River Elbe that houses 50 buildings, some of which are over 400 years old.
The ramparts extend for around 1,800 meters and are up to 42 meters high with steep sandstone faces.
Like any self-respecting fort, it has its own protected water supply, a 152.5 meter deep well. This makes it Saxony's deepest and Europe's second deepest well.
It was used for many centuries as a state prison and is largely intact. Today it is one of the region's most popular tourist attractions with 700,000 visitors a year.
21. Cheyenne Complex - America's underground bunker
Last, but by no means least, on our list of the world's Greatest Fortresses is a, relatively, very modern one. The Cheyenne Mountain Complex is a formerly top-secret military installation built into a mountain in El Paso County, Colorado.
It was built between May the18th 1961 and February the 8th 1966. It used to serve as the center for the United States Space Command and NORAD HQ.
In this role, it monitored North American airspace watching out for missiles, space systems, and foreign aircraft to provide an early warning system for U.S. Military Forces.
This function was moved to the nearby Peterson Air Force Base in 2008 and the complex, today, is used for flight crew training and as a backup command center for NORAD.
The complex is built under over 600 meters of granite over an area of 5 acres. It contains 15 subterranean facilities that are 3 stories tall, each earthquake and explosion proof.
Each of the 15 buildings sits on a bed of more than 1,000 giant springs that are designed to prevent them from moving more than an inch (2.5 cm). It is also certified by the U.S. Department of Defence to be able to withstand an EMP attack thanks to $700 Million shielding works by Raytheon.