What defensive structure did the Greeks fight in front of at Thermopylae?

The Greeks fought in front of the Phocian wall at Thermopylae.

The Phocians had built the defensive wall at the pass’s central gate in the previous century, reportedly to help defend against invasions from Thessaly. When the allied Greek army arrived in 480 BC, the wall was repaired or rebuilt as part of the position against Xerxes’ invasion.

The wall stood at the narrowest part of the pass, where the terrain limited the number of Persian troops who could attack at once. Greek hoplites fought in front of it, using the confined ground to make their shields, armor, and long spears especially effective.

The Phocian wall did not permanently stop the Persians. After Ephialtes revealed the mountain route around the pass, the Greek defenders were outflanked. On the final day, the remaining troops fought on higher ground beyond the wall before being overwhelmed.