The Fortress can be visited
Castell de Bellver

The Castell de Bellver is a unique fortress in Palma de Mallorca. Its special characteristic lies in its unusual circular construction, which sets it apart from most other defensive structures in Spain and Europe.
History+
In 1300, the construction of the Castell de Bellver began under the direction of master builder Pere Salvà, who had previously worked on the Almudaina Palace. Eleven years later, in 1311, the structure was completed and served James II as both a protective castle and a residence. After the lost Battle of Llucmajor in 1349, the fortress became a prison for the widow and children of James III.
Source: Wikipedia · Licence: CC BY-SA 4.0 · Image: https://de.wikipedia.org/wiki/Datei:Castell_de_Bellver_%28Palma%29.jpg
This description draws on the facts documented in the linked Wikipedia article; Wikipedia content is licensed under CC BY-SA 4.0. Wording and editing: Mobile History.
Location: 39.5638, 2.6192