In the Classical times, the settlement of Sounion flourished thanks to the rich mines in the area. It also became one of the largest slave selling centres. The hill was fortified (in 412 BCE according to Thucydides) so Sounion was turned into one of the 5 most important strongholds of the Athenians. In the mid-5th century BCE, by order of Pericles, the magnificent Temple of Poseidon was built, the ruins of which now dominate the region, and the smaller temple of Athena Sounias with the unique layout.
During the Hellenistic period a small settlement grew inside the fortress, serving primarily as a military camp. The decline of Sounion begun at the end of the Hellenistic period. By the Roman times the two temples and the fort had already been deserted and probably abandoned. Pausanias describing the monuments in the 2nd century AD, confuses the temple of Poseidon with the temple of Athena, which indicates the abandonment of the area.
In recent years, Sounion was a popular destination for tourists, many of whom had their names engraved on the ruins of the temple of Poseidon. The most famous signature is that of Lord Byron.