The sanctuary of Poseidon is enclosed by a wall covering an area of approximately 5000 square meters, and the only access was through the monumental gateway, Propylon, with the three entrance-exit passageways on the northern side. The gateway is an oblong building of marble and limestone. The two lateral passages were intended for pedestrians, and the middle one was for vehicles and sacrificial animals. A nearby small building to the west may have served as a guard room. The interior walls were painted red.