Here is what Appian had to say:
“The harbors had communication with each other, and a common entrance from the sea twenty meters wide, which could be closed with iron chains. The first port was for merchant vessels, and here were collected all kinds of ships' tackle. Within the second port was an island which, together with the port itself, was enclosed by high embankments. These embankments were full of shipyards which had capacity for 220 vessels. Above them were magazines for their tackle and furniture. Two Ionic columns stood in front of each dock, giving the appearance of a continuous portico to both the harbor and the island. On the island was built the admiral's house, from which the trumpeter gave signals, the herald delivered orders, and the admiral himself overlooked everything.