Thermopylae, in central Greece, was an ideal place for the coalition-defenders to resist a Persian invasion. An east-west pass,* between the mountains and the sea, was extremely narrow. If this were the place where Xerxes’ forces were to make their way into enemy territory, they would have extraordinary difficulty. Huge numbers of men could accomplish little if they couldn’t get through.
The topography of Thermopylae fit the Greek battle plan for fighting on land: Delay Xerxes and his men as long as possible. If the Persians couldn’t get through, how would they eat and drink? Even if they had tremendous supplies - and even if the army were much smaller than Herodotus tells us - they would still run out of food and water. Scholars believe the Greek plan was reasonable.
Herodotus describes the general terrain which - thanks to Livius.org - we can match with pictures:
The river Asopus ... flows for awhile along the foot of the hills. [This was the general area of the Persian camp.] (Herodotus, Histories, 7.199)
There is also another river called Phoinix, [meaning, “the purple one”] to the south of the Asopus, of no great size...Behind the river Phoinix is the narrowest place, for here has been constructed a road with a single wheel-track only. Then from the river Phoinix it is a distance of three kilometers to Thermopylae. (Herodotus, Histories, 7.200)
This path ... stretches over the ridge of the mountain and ends ... by the stone called Black Buttocks ... where is the very narrowest part. (Herodotus, Histories, 7.216)
Herodotus says the Greek coalition totaled 4,200 men at the start of the battle. Xerxes, naturally, sent a “mounted spy” to report whether the Greeks appeared scared. Astonished, the Persian told his king that he’d seen Spartans exercising and combing their hair!
Xerxes asked Demaratus what such laughable behavior could possibly mean. Demaratus:
These men have come to dispute the pass with us; and it is for this that they are now making ready. ‘Tis their custom, when they are about to hazard their lives, to adorn their heads with care ... Thou hast now to deal with the first kingdom and town in Greece, and with the bravest men. (Herodotus, Histories, 7.209)
Thermopylae - which means “Hot Gates” - takes its name from the hot sulfurous spring nearby. Fighting at the gates would get very hot over the next three days.
Responding to a worried defender, who observed there were so many invaders their arrows would surely darken the sun, Dieneces - a Spartan - famously retorted:
Our Trachinian friend brings us excellent tidings. If the Medes darken the sun, we shall have our fight in the shade.
* Herodotus describes the coastal road as north-south.