The Fortress can be visited
Forte Sangallo
In Nettuno, Forte Sangallo, a Renaissance fortress, rises directly on the Tyrrhenian Sea. This complex attracts visitors who wish to explore the medieval village.
History+
The construction of Forte Sangallo took place in 1501 under Pope Alexander VI and his son Cesare Borgia. The architect is believed to be Antonio da Sangallo the Elder. The main task of the fortress was to protect Nettuno, which was then considered a granary of Lazio, from attacks from the sea. The fortress changed hands several times: after the Borgias, it passed to the Colonna family, then to the Apostolic Camera, and from the 19th century to the Borghese family. In 1931, Forte Sangallo was owned by Baron Alberto Fassini and hosted Queen Marie of Romania and Princess Ileana of Romania that year.
Source: Wikipedia · Licence: CC BY-SA 4.0 · Image: http://commons.wikimedia.org/wiki/File:Forte_Sangallo.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: 41.4566, 12.6560