At the north-western end of the “old city”, we find the magnificent castle made of Apulian limestone with rusticated facing, whose northern side until about a century ago was washed by the sea. Its austere defensive appearance is due to its origins as a means of controlling the territory of Norman power over the conquered cities. Numerous restorations, as well as changing functional uses, have taken place up to the nineteenth century Bourbon period. The castle complex incorporates pre-existing Byzantine structures (10th century) including remains of living quarters and a church, below the floor level. The first Norman and then Swabian architectural nucleus can be identified within the bastioned perimeter of the 16th century walls with four square corner towers, of which three can be admired today: the tower to the north-east called “del Monaco or di San Francesco”, due to a legend that in the 13th century an encounter took place between Frederick II and a Saint; the “dei Minorenni” tower, to the south-west, used as a juvenile prison between the 19th and 20th centuries, and to the south-east, the “Semaforo” tower, for the ancient presence of military naval signaling installations. It was not easy to rebuild by the Norman ruler Ruggero II, especially due to the rebellious nature of the people of Bari, who damaged the building in the 12th century. In the Frederickian era, the structure was expanded and took on a trapezoidal layout, with bugnati curtain walls and defense towers around an elegant central courtyard, which underlined its residential and representative function. To this period belongs the refined double archivolt of the portal, an excellent demonstration of Frederickian Gothic art, with precious low reliefs, among which the imperial eagle with prey between its talons is noteworthy. In the southeast porch of the courtyard, one can admire columns with delightful capitals, with plant and animal figures, signs of an artistic style of eastern Romanesque origin, with Islamic influences. Further restorations were carried out in the Angevin period, especially on the seaward side. Isabella of Aragon and her daughter, Queen Bona Sforza, transformed the castle into an elegant fortified princely residence (16th century), with a bastioned perimeter wall with bastions and a deep moat. Today, the wonderful fortress is one of the city’s highlights, often a fascinating starting point for pleasant walks to discover the charming historic center, and a splendid location for artistic exhibitions or cultural and musical events, and home to the Gipsoteca, where plaster casts that constitute the most distinctive examples of Apulian sculpture are kept.