Today the Pass at Thermopylae is much larger than it was in the days when Leonidas led the Spartans and their Greek allies to to stop the Persians. He was able to hold the pass with 4000 (some estimate as much as 7000) Hoplites, something that would be next to impossible today. You can see the pass here:
In 480 BCE the seas were much deeper and extended to the road you see above. Leonidas only had to cover the ground between the hills to the left and the highway/sea. Far easier to do than it would be today.
Once the pass was encircled Leonidas led his Spartans and the Thespians (they remained with the Spartans and fought to the last man).