From Project Athinai
The Sacred Gate was one of the three main gates of Athens. It was of the courtyard type. A recess gave access to the city doors. Two strong towers protected the entry into this recess which was also flanked by two walls with sentry walks. Intermediate doors were placed in the middle of this courtyard. Then, the extremity of this narrow courtyard, the gate proper consisted of two doors protected by two smaller towers. The Eridanus River flowed out of the city through this gate, sharing it with the Sacred Way.
The Sacred Gate was used for the Procession towards Eleusis. From the Akropolis, participants followed the Sacred Way, went out of the city through the Sacred Gate and continued all the way to Eleusis.
Connolly, Peter and Hazel Dodge; The Ancient City - Life in Classical Athens & Rome, Oxford: Oxford University Press, 1998
Martin, Jacques; Les voyages d'Alix - Athènes, Casterman, 2001