The first fortification walls were probably built in the 6th century BCE and strengthened, as reported by Thucydides, in 412 BCE because of the Peloponnesian War, to ensure the safe passing of the Athenian ships, especially those carrying wheat. Repairs and additions to the wall took place during the Chremonidean War (266-229 BCE). The fort was captured by the Macedonians in 332 BCE, by Demetrius the Besieger in 307 BCE and was turned back to the Athenians in 229 BCE.
The wall covers the hill and surrounds an area of about 35 acres. It was reinforced with towers and gates. The main gate was located in the northwest side and was very well fortified. Very close to it, in the western part of the wall, there were two Neosoikoi (shipsheds) which protected the triremes when not in use, to avoid wear from water and weather. Despite their removal from the sea, they could be extremely fast put ready for battle .
Inside the fort and around the road that connected the Temple of Poseidon with the shipsheds, a small settlement grew, probably Hellenistic, which mostly housed soldiers. Νorth of the shipsheds, there was the port of Sounion, with a small settlement.