The Fortress was built in the 16th C. to defend the port and the town from pirates. The upper part of the keep was completed by the great Michelangelo.
 
Michelangelo’s Fort is one of the biggest that were ever built in those days: it has a square plan, with four large bastions at each corner while the Keep is octagonal in shape; it has buttresses covered in travertine and a moat, that has now disappeared, surrounded it on all sides. The walls are crowned by parapets with larger or smaller openings depending on their purpose: to allow for firing hackbuts or cannons. The Fort could be completely isolated from the rest of the Fortress in order that one could concentrate the final defensive resistance here.
 
The four bastions have the following names: San Colombano, Santa Ferma, San Sebastiano, San Giovanni. The San Sebastiano bastion was provided with an underground corridor, which acted as a secret exit from the fortress towards the mainland. It is supposed that it came out somewhere within the walled city boundary. In the bastion of Santa Ferma, at one time in direct contact with the sea, a small chapel had been built from the very outset in the Saint’s honour, as she was the town’s patron saint. The fortress extends entirely over a vast building dating back to the Roman Imperial Age, perhaps a barracks for the ‘classiari’ (Roman seamen) assigned here to man the fleet and Trajan’s port. The building, which has been partially explored, has turned out to be a vast space almost entirely paved with geometric mosaics which have come down to us almost intact. 

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