The Greek city-states had chosen to defend a narrow pass, or gap, between the mountains of central Greece and the sea.  This pass was at a place called Thermopylae. The pass was part of the route/road into Greece from the north.  Inside the pass, the Greek force now waited.  It was made up of 300 Spartans under their king, Leonidas, and about 6,000 soldiers from other Greek city-states. They faced a Persian army of around 100,000 soldiers.