From Project Athinai
The Dipylon (two towers) Gate (also known as the Thriasian Gate or the Kerameikos Gate) was the largest of the three main gates of Athens. It was the departure point of two roads, one leading to Eleusis (the Sacred Way), the other to the Akademeia gymnasium (Academy).
The Dipylon Gate, built in 479 BC, 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. At the extremity of this narrow courtyard, the gate proper consisted of two doors protected by two additional towers.
The courtyard measured 22 m wide by 41 m deep.
The Panathenaic Procession entered into Athens through the Dipylon Gate.
Connolly, Peter and Hazel Dodge; The Ancient City - Life in Classical Athens & Rome, Oxford: Oxford University Press, 1998
Flacelière, Robert, La vie quotidienne en Grèce au siècle de Périclès, Hachette, 1959
Martin, Jacques; Les voyages d'Alix - Athènes, Casterman, 2001
Travlos, John, Pictorial dictionary of Ancient Athens, Books that matter, New York, 1971, p. 159