From Wikipedia, the free encyclopedia:

The River Cephisus flowed in ancient times through the city. Ancient Athens occupied a very small area compared to the sprawling metropolis of modern Athens. The walled ancient city encompassed an area measuring about 2 kilometers from east to west and slightly less than that from north to south, although at its peak the city had suburbs extending well beyond these walls. The Acropolis was just south of the centre of this walled area. The Agora, the commercial and social centre of the city, was about 400 meters north of the Acropolis, in what is now the Monastiraki district. The hill of the Pnyx, where the Athenian Assembly met, lay at the western end of the city.

http://en.wikipedia.org/wiki/History_of_Athens