Buzz
The essence of military defense is perfectly captured in fortresses, from basic wooden enclosures to sophisticated structures featuring walls, trenches, and turrets. Historically, defenders have always aimed to outsmart the strategies and armaments of their assailants. This relentless pursuit of innovation is evident in the many brilliantly constructed strongholds that exacted a heavy toll on any invading force.
Below is a compilation of ten such fortresses, varying from formidable to virtually unconquerable.
10. Masada
The Defenses: Masada's near-impenetrability stemmed from the towering elevation of its solitary plateau. Soaring 1,424 feet (434 meters) above the barren desert landscape southwest of the Dead Sea, it was further reinforced by the Hasmoneans in the 2nd century BC, later becoming the site of Herod the Great's palace.
Herod fortified the mesa's edge with a 4,500-foot-long (1,372-meter) double-layered defensive wall, nearly 20 feet (6 meters) tall. This wall featured over 30 towers and four gates. He also built aqueducts capable of delivering 200,000 gallons (757,082 liters) of water to the cisterns. Combined with well-stocked storerooms, these measures ensured that up to 10,000 defenders could endure a siege for several months.
Was It Conquered?: Following Jerusalem's fall in the Jewish revolt, rebel holdouts continued to resist the Romans from Masada. In AD 72, the Roman Xth Legion was sent to capture the fortress. Approximately 15,000 Romans laid siege to around 1,000 Jews, including women and children. They constructed a massive ramp of stone and earth to transport siege engines to the summit. Upon breaching the wall, the defenders chose mass suicide over surrendering to Roman enslavement.
9. The Theodosian Walls
The Defenses: When the empire's capital shifted to Constantinople, Emperor Theodosius II initiated the construction of its defensive walls, which were enhanced over centuries. The first barrier was a 65-foot-wide (20 meters) and 23-foot-deep (7 meters) ditch, which could be flooded. Beyond this lay an outer wall, followed by a second wall equipped with towers and firing platforms.
Further in stood a third wall, 16 feet (5 meters) thick and 39 feet (12 meters) high, boasting 96 projecting towers, each capable of housing three artillery pieces. Built on an elevated embankment, the design prevented siege engines from reaching the inner wall. From these imposing defenses, the protectors could unleash their secret weapon—Greek fire—upon advancing enemies.
Was It Conquered?: For eight centuries, Persians, Slavs, Avars, Rus, and Bulgars relentlessly attacked the walls without success. In 1204, the Fourth Crusade managed to seize Constantinople, but only because a gate was inadvertently left open. Although the city was returned to Byzantine control, the empire was already weakening. By 1453, when Ottoman Sultan Mehmed II besieged Constantinople, it was defended by a mere 7,000 men against an army of 50,000 to 80,000.
Mehmed brought the largest cannons ever seen at the time. The most formidable, the Royal Gun, required 60 oxen and 400 men to transport. Its stone projectiles weighed 1,500 pounds (680 kilograms). Despite this overwhelming firepower, Mehmed grew so frustrated with the stalwart defense that he considered abandoning the siege, offering to withdraw in exchange for tribute. The Byzantines refused, forcing Mehmed to continue his relentless assault until the walls finally crumbled. After 53 days of relentless bombardment, the Ottomans reclaimed Constantinople.
8. Kenilworth Castle
The Defenses: Constructed in the late 11th or early 12th century, Kenilworth in Warwickshire is renowned for its water-based defenses and multi-layered walls. Originally a motte and bailey timber structure (a motte being a raised mound with a keep, and a bailey a walled courtyard), it was later transformed into a stone fortress, further reinforced by King John between 1210 and 1216.
The Great Keep featured “fishtail” arrow slits, a groundbreaking design enabling crossbow fire. Nearby streams were dammed to form an artificial lake, the Great Mere. Surrounding the castle on all sides except the north, this lake, the largest man-made water defense in England, rendered the fortress nearly impregnable.
Was It Conquered?: During the barons' revolt against King Henry III, Kenilworth, held by rebel Simon de Montfort, endured a siege by royal forces from June to December 1266—the longest in English history. Prince Edward attempted to breach the defenses using heavy trebuchets and nine to eleven siege engines, but without success. Even a nighttime assault by water was thwarted by the Great Mere. Ultimately, it was not military might but disease that forced the castle to surrender.
7. Mont St. Michel
The Defenses: Mont St. Michel's natural setting is already daunting. Perched on a rocky tidal island off Normandy's coast, it is accessible only on foot during low tide. The tides are notoriously powerful and hazardous, with a typical 50-foot (15-meter) difference between high and low tides. At times, the tides shift so rapidly that the bay transforms into quicksand. Beyond nature's defenses, French kings Philip II and Louis IX reinforced the island with additional walls and fortifications.
Was It Conquered?: Mont St. Michel's defenses were continuously enhanced throughout the Hundred Years' War. Although the church's chancel collapsed during the intense siege of 1421, the citadel, defended by a handful of knights, withstood English assaults for three decades.
6. Krak des Chevaliers
The Defenses: Located near the Syria-Lebanon border in the former Crusader county of Tripoli, the Krak utilized cutting-edge 13th-century engineering to counter the increasing power of siege weaponry, such as counterweight trebuchets. It featured larger towers, thicker double walls with sloped exteriors to resist breaches, and advanced defensive mechanisms.
Machicolations, loopholes, arrow slits, and “murder holes” provided protected openings for defenders to launch projectiles without exposing themselves. The Crusaders leveled the rocky summit, eliminating any footholds for attackers. The main gateway was accessible only through a sharp hairpin turn, slowing enemy advances. Clever lighting adjustments further disoriented invaders.
Was It Conquered?: In 1271, Mamluk leader Al-Zahir Baybars laid siege to the Krak. His catapults demolished an outer tower, and it took two weeks to breach the inner wall. The remaining Knight Hospitallers held out for 10 more days in the inner wall's largest tower until a forged letter, supposedly from their master, ordered their surrender.
5. Kumbhalgarh
The Defenses: Situated in Rajasthan, northern India, Kumbhalgarh sits atop mountains 3,560 feet (1,087 meters) above sea level. Its defensive walls stretch 20 miles (32 kilometers), with widths ranging from 15 to 25 feet (4.5 to 7.6 meters)—wide enough, it is said, for eight horses to gallop abreast.
Access is granted through a series of sharply angled ramps, challenging to navigate. Seven massive gates, fitted with enormous iron doors adorned with spikes, were designed to deter attacking elephants. Even if invaders breached these, they faced narrow pathways, five-foot-high (1.5-meter) doors, and a maze of deceptive traps within the fort.
Was It Conquered?: Since its construction in the 15th century, the fort remained unconquered through countless assaults. However, in 1576, Emperor Akbar's deceitful general, Shahbaz Khan, poisoned the water supply, forcing the defenders of Kumbhalghar to choose between surrender and death by thirst.
4. Hochosterwitz
The Defenses: Perched atop a 564-foot (172-meter) dolomite rock in Austria's Carinthia province, Hochosterwitz Castle is accessible only via a serpentine path obstructed by 14 fortified gates. Attackers had to conquer each gate sequentially while enduring ambushes from defenders. Beyond this primary defense, the castle features a traditional square tower and protective walls.
Was It Conquered?: The gates were built between 1571 and 1600 during Turkish invasions into Austria. Despite their efforts, the Ottomans never advanced beyond the fourth gate. Hochosterwitz remained unconquerable and is still owned by the same family today.
3. Switzerland
The Defenses: Essentially, the entire nation functions as a fortress, shielded by the towering Alpine mountains. Known as the Swiss National Redoubt, it comprises a network of bunkers, tunnels, shelters, warehouses, and military installations concealed within the mountains. During the Cold War, roads, bridges, and railways were equipped with explosives to ensure their destruction, rendering them useless to invaders.
Roadside mountainsides were rigged with traps to trigger artificial avalanches. The population could retreat to thousands of bomb shelters for extended periods. Additionally, all able-bodied men are still required to undergo military training. During World War II, they were ordered never to surrender in the event of a German invasion and, if necessary, to retreat to the mountains and engage in guerrilla warfare.
Was It Conquered?: The Nazis' disdain for Switzerland led to Operation Tannenbaum, a plan to invade the country. However, the anticipated cost of subduing the Swiss deterred them from executing the plan. To this day, no one has dared to challenge Switzerland.
2. Murud Janjira
The Defenses: Situated on an island in the Arabian Sea, Janjira Fort is an oblong structure accessible only by sea. Attackers faced 40-foot (12-meter) high walls reinforced with 19 bastions and over 500 strategically positioned cannons. Among these were three of India's largest cannons, including Kalal Bangadi, weighing 22 tons and capable of firing projectiles up to 7 miles (12 kilometers).
Was It Conquered?: Maratha ruler Chatrapathi Shivaji attempted to capture the fort 13 times between 1659 and 1678 but failed. His son Sambhaji nearly succeeded by constructing a causeway from the shore, but he was diverted by an attack on his own fort and had to withdraw. Despite efforts by the Portuguese, Dutch, and British, Janjira remained unconquered.
1. Castillo de San Marcos
The Defenses: Constructed by the Spanish from 1672 to 1695 in St. Augustine, Florida, San Marcos is the oldest surviving coastal fort in the continental U.S. Its star-shaped bastion design, a response to the vulnerability of traditional square forts to cannon fire, reduced the surface area exposed to direct hits. This design also allowed defenders to fire at attackers from multiple angles, including the sides and rear.
San Marcos had an additional advantage: its walls were made of coquina, a local limestone formed from coquina clam shells. The stone's microscopic air pockets made it compressible, though the Spanish were unaware of this. They built the walls 35 feet (10.7 meters) high and 12 to 19 feet (3.7 to 5.8 meters) thick, believing the soft stone to be a weakness. This feature later proved to be a surprising strength.
Was It Conquered?: In 1702, British forces from South Carolina besieged St. Augustine, setting the town ablaze and bombarding the fort. However, their cannonballs either bounced off or sank harmlessly into the soft coquina walls. The porous stone absorbed the impacts, rendering the attack ineffective. After 50 days of futile efforts, the British withdrew.
They attempted another siege in 1740, achieving the same outcome. Castillo de San Marcos was never conquered by force; it was eventually handed over to the British through a treaty following the Seven Years’ War.
