Bastion de l'Etendard is the main museum inside Bonifacio's citadel. It uses the old defensive works to tell the story of a town shaped by sieges, sea power, and its exposed position above the strait.
It is worth pairing with a walk through the upper town. The museum gives context to the walls and lanes around you, then sends you back outside to see Bonifacio less as a postcard cliff town and more as a lived-in fortress.