The Fortress can be visited
Château de Brest

In the French region of Brittany, specifically in Brest, Finistère, stands the Château de Brest. As the oldest monument in the city, it is strategically located at the mouth of the Penfeld River, controlling one of the world's largest natural harbors. Its architecture shows over 1700 years of military use and continuous adaptation to warfare, making it one of the oldest continuously used fortresses worldwide.
History+
The site's origins date back to the Stone Age, but the first verifiable fortification comes from the Roman era, at the end of the 3rd century. The Romans erected a castrum with up to ten cylindrical towers to defend against Saxon raids.
Source: Wikipedia · Licence: CC BY-SA 4.0 · Image: http://en.wikipedia.org/wiki/File:21-09-2007-tourparadisbrest.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: 48.3812, -4.4943