Alfonsino Castle, known also as Red Castle because of the vermilion colour of its stone, sits on Sant'Andrea Island at the entrance to Brindisi’s harbour, in front of the Pigionati Channel.

Construction work began in 1481 under Alfonso II of Naples, who built a tower to guard the port and to serve as a defensive outpost for the city. A few years later, Alfonso of Aragon transformed the original defensive structure into a proper castle.

The Castle was built on a strip of land surrounded by the sea, where there had previously been a Benedictine abbey dedicated to Sant'Andrea (it was destroyed in the 15th century). Next to it is the Forte a Mare (Sea Fortress), which was built in the 16th century to serve as a barracks for the troops.

Nowadays, the Castle can be visited with authorization by the Navy. 

The original structure, a watchtower, was subsequently incorporated into a genuine castle, fitted with formidable ramparts and bulwarks. The Castle, which was isolated when the canal was later opened, was completed in 1492 with the creation of the great hall and two galleries. Construction on the Fort began in 1558, while the two bastions were added in 1577.

An archivolt leads into the castle's distinctive internal port. The Alfonsino castle, an amazing example of 16th-century fortified architecture, is divided into two main sections with the Castle on one side, and the Fortress on the other. The horn-shaped fortress, which was built by Philip II of Austria in 1583 to guard the Castle and which was later used to house the garrisons, encloses and protects the island.