Located at the mouth of the river Penfeld, the Château de Brest is the oldest monument in Brest. Strategically placed, this military fortress has over 1,700 years of history in its original role - from the Roman castellum to Vauban's citadel that stands proudly on the opposite river bank to the Tour Tanguy.
Classified as a monument historique in 1923, it is the oldest castle still in use with the French Navy still operating here. Today, it is also home to the National Naval Museum with lots of exhibits looking at the history of the Brest arsenal and has recently begun showing some shipbuilding exhibits in its dungeon.
http://www.brittany-ferries.co.uk/guides/france/brittany/brest/attractions/chateau-de-brest