According to archive sources, the castle “dei Paleologi” at Aqui Terme already existed in the 11th century when it was under Episcopal control. Gugliemo VIII Paleologo, marquis of Monferrato rebuilt it during the second half of the 15th century. The late medieval complex comprised the inner fortification, on the hill summit, protected by a ditch and outer curtain wall. The castle was damaged and underwent several transformations in the modern period until, in 1787, the Sardinian administration decided to turn the castle into a prison (it kept this function until recent years) and this entailed the construction of new buildings – incorporating part of the external curtain wall, on the south-east side towards the town. The intention to increase the present use of the complex for cultural activities (it houses the Civic Archaeological Museum and related storage facilities) has led the local administration to undertake the construction of an underground multifunctional hall below the courtyard between the prison and the castle.

The rescue excavation, undertaken between October 2004 and January 2005, revealed traces of the various occupation phases of the castle hill.

No primary traces (structures or occupation surfaces) of the Roman period settlement were found, only pottery in secondary deposition in the colluvial layer. This may provide indirect confirmation of the hill’s occupation in antiquity, today no longer verifiable due to the substantial medieval and modern interventions, but suggested by various traces.
An imposing wall belonged to the 15th century (period B) fortifications. Built of bricks and strong mortar, it had the same characteristics as the earliest sections of the preserved standing walls. This structure constituted the foundation of the curtain wall built against the slope with defensive as well as containment functions.

In the 16th-17th century (period C), the area saw the construction of a rectangular service room and a well in the space between the exterior curtain wall and the ditch. The final phase was constituted by the structural modifications for the prison construction at the end of the 18th century (period D), still standing. A number of stone buttresses abutting one wall of the 16th century building date to this phase.