In 437BC, a new gateway to the Acropolis, the Propylaia, was begun by the architect Mnesicles. This building was conceived as a monumental backdrop to the conclusion of the Panathenaic procession, an architectural transition from the profane world of the lower city to the sacred ground of the gods watching over it. It was approached by a massive stepped ramp 80 metres (262 feet) long which rose 25 metres (82 feet).

Situated at the edge of the plateau, the Propylaia negotiates an abrupt change in elevation. It comprises two temple-like, open porticoes at different levels - the western portico being lower to align with the end of the ascending ramp. The eastern portico, separated by a series of steps, led through five doorways (pylai-hence the plural Pro-pylaia). The western portico channeled the procession through a narrower passage, defined by a double row of Ionic columns. It is thought Mnesicles chose the Ionic order for symbolic reasons (the Athenians being Ionian Greeks), but also because the more slender proportions allowed him to reach the beams supporting the building's taller ceiling without crowding the interior space. Interestingly, the Propylaia's porticoes were reinforced with iron beams in the architraves.

Flanking the Propylaia's western portico are two projecting, colonnaded wings. These appear to be equal in size but are in fact quite different. The left or northern wing housed the Pinakothiki, a picture gallery thought to have served also as an official dining hall. The south wing is an open, shallow screen intended to mirror the Pinakothiki, creating a symmetrical composition around the entrance portico. Perhaps Mnesicles had intended to extend the building equivalently on this side. If so, he altered his plans so as not to encroach on the site of the little Temple of Athena Nike. He seems also to have deleted from his plans two large spaces intended to flank the eastern portico. Most likely work was suspended due to the outbreak of the Peloponnesian War in 431 BC, which pitted Athens and her allies against Sparta. The Propylaia is clearly an unfinished building. This is particularly evident in the many untreated stone surfaces which even preserve the projecting bosses that were used to hoist the blocks into place.