From Project Athinai
The Desmoterion (State Prison), where Socrates would be executed in 399 BC, was built around 450 BC outside the Agora, 60 m to the southwest of its southwest corner, along the south side of a street leading to the Peiraeus Gate.
The building had a foundation of large poros blocks and measured 17 m wide by 40 m deep. From the entrance, a central corridor led to an office at the northeast corner and to three square cells on the east, to five more cells on the west and to a large walled courtyard on the south. The office consisted of 4 rooms and, possibly, of more rooms on an upper floor. There was a basin in the floor of the nortwest cell and a large water jar has also been found in the same cell.
American School of Classical Studies at Athens; The Athenian Agora: A Guide to the Excavation and Museum, 1990