Despite the scattered clues for prehistoric occupation of Sounion and its mention in Homer's Odyssey, definite religious activity at this site begins only in the 7th century B.C. The evidence for this religious activity takes several different forms, including votive offerings, which verify the presence of two separate sanctuaries, one of Poseidon at the southernmost tip of Attica and the other of Athena about five hundred meters to the northeast. The excavation of Sounion has not revealed whether a temple was built in either of these sanctuaries during the 6th century B.C, but it does show that large-scale religious architecture was constructed in the beginning and middle of the 5th century B.C. Precise dating, however, is unfortunately not available for most of the monuments at Sounion, and thus defining an exact building sequence is not possible.
Located at the southernmost and highest part of Cape Sounion, the sanctuary of Poseidon unequivocally illustrates a powerful connection between architecture, topography and divinity by its imposing structure and dramatic placement at the edge of steep cliffs overlooking the sea. The oldest extant remains of a temple for Poseidon belong to a poros temple constructed during the 490's or 480's B.C., destroyed before it was ever completed when the Persians invaded Attica in 480 B.C. Following the Persian destruction and prior to the construction of a new temple, a temporary shrine may have been constructed, as indicated by a small structure to the south of the older temple. The rubble walls of this provisional temple are scarcely preserved, but the poros column drums from the older temple seem to have been recycled here. A slightly larger Periklean temple (its stylobate measuring 13.47 by 31.12 meters, as opposed to the 13.06 by 30.20 meters of the older temple) of Agrileza marble, quarried from the neighboring mountain, was built in 450-440 B.C. atop the late Archaic temple foundations. This temple helped to develop the canon of the 5th century B.C. Doric order, for many of its features became more or less standard in the following years. One of these features was its external columnar arrangement (the same as its late Archaic predecessor), consisting of six columns on the east and west façades and thirteen running along the north and south sides. Certain Archaic features, such as variation in the width of spacing between columns (intercolumniations) and in the size of the columns themselves (as measured from their diameters at the base), were abandoned in this temple (except at the corners), and thus it served as an important precedent for early Classical temple architecture. The columns on this temple are unusual in that they have no entasis, a rather striking characteristic considering that this temple was contemporary with the Parthenon and the Propylaia, each of which included columns designed with significant entasis. Also, each column had only sixteen flutes, not twenty like most Doric columns, a peculiarity probably attributable to the relative softness of the Agrileza marble (Tataki 1978: 34). The edges (arrises) of the sixteen flutes were presumably carved more bluntly so as to be less vulnerable to wear.
The Temple of Poseidon featured various kinds of sculptural decor, preserved to different degrees, in the form of pedimental sculpture, a continuous Ionic frieze, and akroteria. Although the pedimental sculpture is lost except for one headless, seated woman, a virtually complete floral akroterion survives that would have crowned the pediment. The frieze, which ringed the rectangular interior space in front of the temple's front porch (pronaos), portrayed the common sculptural themes of the Gigantomachy, the deeds of Theseus, and the Centauromachy.
This building is attributed by some to the 5th century B.C. architect who is also credited with the Temples of Hephaistos in Athens, Ares at Acharnai, and Nemesis at Rhamnous. The association of these four temples is due to their proportions (i.e. extremely thin columns below relatively heavy entablature), their employment of two different materials of surface treatments for the three steps, and other structural and stylistic features such as the alignment of the antae and columns of the pronaos with the third columns of the exterior colonnade's long sides.
The sanctuary of Poseidon was entered from the north through the monumental propylaia, an impressive gateway of poros and marble comprised of two Doric porches of unequal length. Both porches contained two columns in antis and were separated by a gate wall with three doorways, the widest of which was in the middle. This feature would have been conducive to leading animals in procession. Along the north side of the peribolos was a small building, probably a guardhouse (Tataki 1978: 32), that shared its east wall with the gateway and had an opening facing the temple. Directly to the west of this building was a stoa, the rear wall of which bounded the temenos on the remainder of the north side. This poros stoa, built after the propylaia (Tataki 1978: 32), measured 25 by 9 meters and contained two rows of columns. One row ran lengthwise down the center with six columns that may have been reused from the late Archaic Poseidon temple (Tataki 1978: 32), while another row with eight or nine columns stood on the façade. An additional smaller stoa ran along the west side of the temenos and was connected to the larger, earlier stoa in the northwest corner of the sanctuary. All of these buildings were neatly combined in an almost square arrangement that distinguishes this sanctuary as an early example of regularized temenos design.
Although the southern tip of the cape is dominated by the sanctuary of Poseidon, this area also functioned as an important fortress. When this function was established, however, is a matter of debate. The original excavator believed it to be a 6th century fortification, yet most modern scholars date the wall to the end of the 5th century during the Peloponnesian War, at which point it would have been necessary to control the passage of ships through this area. Regardless of its initial construction date, many modifications were made over the years, explaining the presence of various materials and building techniques. The wall itself is 3.5 meters wide by 300 meters long and begins at the northeast corner of the sanctuary, runs north and then turns west toward the coast. Eleven square towers strengthened the wall, as did a bastion that was built in the 3rd century B.C., probably at the same time as other additions and improvements.
Five hundred meters to the northeast of the sanctuary of Poseidon was the smaller sanctuary of Athena Sounias, consisting of a temple to Athena, another smaller temple of questionable identification, and a trapezoidal circuit wall which predated and enclosed these two buildings. The Ionic Temple of Athena was built on a smaller scale (19 by 14.5 meters) than its neighbor within the sanctuary of Poseidon, but it was built of the same brilliant white marble from Mt. Agrileza. Inside the cella were four Ionic columns, serving to support the roof. Also inside was the cult statue, its position still marked today by the foundations of its pedestal that stands behind the interior columns at the west end of the temple. Several peculiarities of this structure explain why the Roman architect Vitruvius included it some four centuries later in his list of "abnormal" temples. The most conspicuous peculiarity is that it was semi-peripteral with its outer colonnade stretching only around the east and south sides (with ten and twelve columns respectively) and with just two columns on the north and west sides to intersect the cella wall. The omission of north and west colonnades was not architecturally imperative, but rather indicates that the temple was built to be viewed principally from the south and east. And indeed these are the directions from which the temple would have been seen from a ship approaching the cape (Scully 1979: 162). A further peculiarity of this temple is that the altar for Athena was located on the south side of the temple, as shown by a flat cutting in the rock. This feature is an additional indication that the most meaningful experience of this temple was from the south and east sides, for the worshippers would have had to pass by the east façade on their way to the southern altar.
Exactly when or how this building was built is a matter of contention. Some believe it was constructed in 470 B.C., while others place its erection during the time of the Peloponnesian War. An entirely different argument is that is was constructed in two parts, first the cella in the Archaic period, and later the external colonnade, a new roof and the internal columns necessary to hold up this new roof.
The only two columns in the small Doric prostyle temple lying northest of the Ionic temple in the sanctuary of Athena were on the east façade and are preserved at the level of their square foundations. Within this building the Eleusinian limestone base of a kouros still remains, as does part of the altar just to the east of the entrance. These finds, however, cannot furnish an identification for this temple that was constructed contemporarily with or slightly after the larger temple in the same sanctuary. While it is possible that this was another temple to Athena, it is also conceivable that it was dedicated to Artemis (Tataki 1978: 42).