The city of Babylon was essentially a giant fortress back in the fifth century B.C. The city had two layers of walls, so if the first layer was taken, a second wall would stop any attacker. In between these two walls was a moat that made taking the city even more impossible. Watchtowers filled with archers jutted from the walls, allowing them to rain down arrows on anyone who tried to approach them.
Herodotus, a Greek historian, estimated the walls to be 300-feet high, 80-feet wide and 55-miles long (though modern estimates peg them at 90-feet high and 10-miles long, so either Herodotus liked to embellish, or he was horrible at measurements). Also, the city of Babylon was bisected by the Euphrates River, which made sure the city had a constant supply of water in the event of a siege. This will be important later.
Babylon was the heart of the Neo-Babylonian Empire that stretched from the Mediterranean to the Persian Gulf, and in 539 B.C., Persia, under King Cyrus, came calling (this would be about 60 years after the events of 300).
Cyrus, rockin' a sweet beard of power.
The Fatal Flaw:
Cyrus marched toward the city with his army. The soldiers of Babylon were so unconcerned about the city being taken that they were busy partying during a harvest festival, paying little attention to the Persians.
We mentioned that the city straddled the Euphrates river. The massive walls of the city also went across the river, with underwater gates allowing the current to flow through. Babylon had no defenses at these gates because, what would the enemy do, send an army of dolphins after them? Nobody could hold their breath long enough to go down there and break through the gates.
Not even this could help.
Cyrus's plan? Get rid of the water. He diverted the Euphrates itself into canals and reservoirs, which lowered the river's water level in the city. The gates were suddenly exposed and, in the darkness of night a few Persian soldiers were able to swim/wade through the openings and open the city gates for the rest of the Persian army.
By the time the Babylonians figured out what was going on, it was too late. They mounted a counterattack but they were drunk from partying and were quickly slaughtered.
King Cyrus continued his conquest of the region and eventually the Persian Empire became the largest empire in the ancient world.
Before the battle of Singapore in World War II, the British were pretty sure their stronghold was invincible. To the north of the city were impassable swamps and jungle that no one was crazy enough to try to march an army through. The south was protected by five 15-inch guns (that is, the shells were 15 inches across) that would turn approaching ships into Swiss cheese, and 85,000 British troops to meet anyone who landed.
The British called it the Gibralter in the Far East and their main concern was that it was too untouchable. As one officer in the British army was quoted as saying, "I do hope we are not getting too strong in Malaya because if so the Japanese may never attempt a landing."
The Fatal Flaw:
The whole defense of the city was dependent on the idea that the attack would come from the south sea. After all, you'd never get an army through the jungle to the north. How would you get your heavy artillery or tanks through it? What were they going to do, put their troops on bicycles or something?
Yes. Thousands and thousands of bicycles. The Japanese came pouring through the jungles with alarming speed. The British turned their coastal guns around and bombarded the Japanese attack, but it accomplished nothing. They were designed to punch holes in ships, not repel a gigantic militarized Tour De France.
The Japanese were able to attack several areas of the island from the north. Fearing the speed of the BMX-riding Japanese, the British detached forces to all these places to avoid encirclement, which spread their lines too thin. These lines were soon broken and they were surrounded. By February 15, 1942, the British had no option other than surrender. The 80,000 men became the largest surrender of British personnel in history.
When recounting the Battle of Singapore, One Australian officer stated, "The whole operation seems incredible: 550 miles in 55 days - forced back by a small Japanese army of only two divisions, riding stolen bicycles and without artillery support." Well, damn, when you put it that way...