It was around the fateful year one thousand that the inhabitants of the area around today’s Sermoneta climbed this hill to settle in a safer place where they could defend themselves from the constant assaults of the Saracens. In fact, at the end of the 10th century a first fortified post was built here: the site was owned by the Church, which granted it to the Counts of Tusculum who then ceded it, in the first half of the 13th century, to the Annibaldi (a family of Germanic origin). It was the Annibaldi who built the first real fortress (it was mainly made up of what is now the castle keep); this family, in turn, sold Rocca and Borgo to the Caetani family who greatly strengthened it.
Everything was stolen from the Caetani by Pope Alexander VI-Borgia who made this castle an important stronghold for the affirmation of the power of his family, an advanced garrison towards the southern border of the papal possessions. On the death of Pope Borgia, Guglielmo Caetani reacquired everything to his family which remained closely linked to Sermoneta in the following centuries.
The Castle, which dominates the medieval town of Sermoneta, certainly originated from the first Rocca degli Annibaldi, was significantly enlarged in particular by Onorato III Caetani (with the major interventions of 1455) which was followed by the renovations commissioned by the Borgias. Antonio da Sangallo the Elder was also involved in the great strengthening of the works for the defense against ‘modern’ artillery weapons.
It is said that Cesare Borgia – true to his fame – as the first act of the restructuring, destroyed the sepulchral chapel of the hated Caetani and had their bones scattered. The Borgias elected Sermoneta as a Duchy which was entrusted to Rodrigo, (Son of Lucrezia and her beloved husband Alfonso di Bisceglie).