Ancient Greek religion was essentially propitiatory in nature: i.e., based on the notion that to avoid misfortune, one must constantly seek the favour of the relevant gods by prayers, gifts and sacrifices. To the ancient Greek, every natural feature, e.g. hill, lake, stream or wood, was controlled by a god. Thus a person about to swim in a river, for example, would say a prayer to the river-god, or make an offering to that god's shrine, to avoid the chance of drowning. The gods were considered immortal, could change shape, become invisible and travel anywhere instantaneously. But in many other respects they were considered similar to humans. They shared the whole range of human emotions, both positive and negative. Thus, in their attitudes towards humans, they could be both benevolent and malicious. As humans also, they had family and clan hierarchies. They could even mate with humans, and produce demi-gods.

Temple of Poseidon at Cape Sounion, built circa 440 BC. 
Greek hexastyle temple with Doric columns. Temple of Hephaestus, Acropolis of Athens. Built c.450 B.C. 
Bronze statue of deity, prob. Poseidon, about to hurl (missing) trident. Height: 2.1m. c.460 B.C. Found in shipwreck off Cape Artemisium. Athens National Archaeological Museum 
Poseidon with trident on Corinthian plaque. c.550 B.C. Louvre, Paris. 
Byron's name carved into temple of Poseidon, prob. 1810. 
George Baron Byron posing in Albanian costume. Painted by Thomas Phillips, 1813.In a maritime country like Greece, the god of the sea was bound to occupy a high position in the divine hierarchy. In power, Poseidon was considered second only to Zeus (Jupiter), the supreme god himself. His implacable wrath, manifested in the form of storms, was greatly feared by all mariners. In an age without mechanical power, storms very frequently resulted in shipwrecks and drownings.

The temple at Sounion, therefore, was a venue where mariners, and also entire cities or states, could propitiate Poseidon, by making animal sacrifice, or leaving gifts.

The temple of Poseidon was constructed in c.440 B.C., over the ruins of a temple dating from the Archaic Period. It is perched above the sea at a height of almost 60m. The design of the temple is a typical hexastyle i.e. it had a front portico with 6 columns. Only some columns of the Sounion temple stand today, but intact it would have closely resembled the contemporary and well-preserved Temple of Hephaestus on the Acropolis, which may have been designed by the same architect.

As with all Greek temples, the Poseidon building was rectangular, with a colonnade on all four sides. The total number of original columns was 42: 18 columns still stand today. The columns are of the Doric Order. They were made of locally-quarried white marble. They were 6.10 m (20 ft) high, with a diameter of 1 m (3.1 ft) at the base and 79cm (31 inches) at the top.

At the centre of the temple colonnade would have been the hall of worship (naos), a windowless rectangular room, similar to the partly intact hall at the Temple of Hephaestus. It would have contained, at one end facing the entrance, the cult image, a colossal, ceiling - height (6m) bronze statue of Poseidon. Probably gold-leafed, it may have resembled the sculpture, which was created in the same period as the temple. Poseidon was usually portrayed carrying a trident, the weapon he supposedly used to stir up storms.

Archaeological excavation of the site in 1906 uncovered numerous artefacts and inscriptions, most notably a marble kouros statue and an impressive votive relief,[14] both now in the Athens National Archaeological Museum.

The famous inscription of George Lord Byron was presumably carved on his first visit to Greece, on his Grand Tour of Europe, before he acquired fame as a poet. Byron spent several months in 1810-11 in Athens, including two documented visits to Sounion. Byron mentions Sounion in his poem Don Juan:

Place me on Sunium's marbled steep, 
Where the waves and I can only hear, 
Our mutual murmurs sweep... 

Byron, a passionate philhellene, returned to Greece in July 1823, to support the Greeks in their struggle for freedom from the Ottoman Empire (the Greek War of Independence). Based at Messolonghi (Acarnania, western Greece), he spent a substantial part of his own fortune on equipping a fleet and an army. He planned to lead an assault on the crucial Turkish - held fortress of Lepanto (Navpaktos, Aetolia), which dominated the entrance to the Gulf of Corinth. But his plan was thwarted by his death from a fever at Messolonghi in April 1824, aged 36. Byron did not visit Attica during this period.