As soon as the building of the city walls had been completed, Themistocles resumed the construction of the Peiraeus defences, which protected the larger harbour of Cantharus on the west and the smaller ports of Zea and Munychia (respectively southwest and south-east of the Munychia heights), terminating in moles at their entrances and enclosing the entire promontory on the land and sea sides except a portion of the south-west shore of the peninsula of Acte.
In the centre was the Agora of Hippodamus; on the western margin of the Cantharus harbour extended the emporium, or Digma, the centre of commercial activity, flanked by a series of porticoes; at its northern end, near the entrance to the inner harbour, was another Agora, on the site of the modern market-place, and near it the µa?cp l OTOa, the corn depot of the state.
The fine marble lion of the classical period which stood at the mouth of the Cantharus harbour gave the Peiraeus its medieval and modern names of Porto Leone and Porto Draco; it was carried away to Venice by Morosini.
The most stirring episode in the history of the Peiraeus is the seizure of Munychia by Thrasybulus and the exiles from Phyle, and the consequent destruction of the "30 tyrants" in 404 B.C. The three chief arsenals of the Peiraeus were named Munychia, Zea and Cantharus, and they contained galley slips for 82, 196 and 94 slips respectively in the 4th century B.C.