The development of Portus is best understood in terms of five principal phases. The port was established with a large basin and limited infrastructure under Claudius. It was subsequently substantially enlarged with the addition of a new hexagonal basin and a range of warehouses and other public buildings under Trajan. After this, there were major periods of development under Severus and through into the 4th century AD. The commercial life of the port began to contract in the later 5th century AD, while active steps were taken by Byzantine authorities to defend the port in the early to middle 6th century AD.