300 - Narrow Passage at Thermopylae

Leonidas chose to confront the invading Persians at Thermopylae because of the very narrow passage which existed at the scene of battle.  This drawing depicts how the scene may have looked at the time. 

The only way the Immortals could penetrate the ranks of the defenders was to find a different path - behind Thermopylae's seaward passage.

They were given that opportunity by a traitor, named Ephialtes.