A small French colonial fort at the centre of town, built in the 1900s when French Equatorial Africa was being secured and used until independence as a remote outpost. Square walls of mud brick, a single tower, a parade ground inside — modest by the standards of Saharan forts further east, but the only piece of formal architecture in Bilma.
It is now used by the Nigerien military, which limits how close you can get; the exterior and the view of it from the surrounding streets are easy to take in. Ask your guide before pointing a camera at it.