The Fort can be visited
Bréhon Tower

On a rock in the Little Russell Channel off the coast of Guernsey stands the Bréhon Tower. This structure, accessible only by boat, once served to protect shipping lanes and the harbor of Saint Peter Port.
History+
The origins of the Bréhon Tower date back to 1744, when an obelisk was erected on the rock as a sea mark. Due to poor visibility, this was replaced in 1824 by a 40-foot-high tower. In the 19th century, in light of renewed tensions with France and the strategic importance of the Channel Islands, plans for a fortification on Bréhon became concrete. Construction of the current Bréhon Tower began in 1854 and was completed in 1857 under the direction of Thomas Charles de Putron. The fortress was intended to monitor shipping traffic between Guernsey and Herm and secure the harbor of Saint Peter Port.
Source: Wikipedia · Licence: CC BY-SA 4.0 · Image: http://en.wikipedia.org/wiki/File:Brehon_Tower.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: 49.4712, -2.4880