The fortress museum is housed inside Fort Thüngen — the surviving Three Acorns redoubt that gave the Kirchberg plateau its earlier name. The permanent exhibition walks you through the construction, sieges, and final dismantling of Luxembourg's great fortress, with original cannon, plans, and a working scale model of the city as it stood in 1865.
Free entry. Combined with a visit to Mudam next door, this is half a day on Kirchberg with a strong contrast between contemporary art and military history.