The Ulm Federal Fortifications are the remains of a huge nineteenth-century fortress system built for the German Confederation, at one time among the largest fortified complexes in Europe. Rather than a single castle, it is a ring of walls, towers, barracks, and outworks spread across Ulm and Neu-Ulm.
The fortifications reward travellers who like urban history in layers. Some sections are reused, some are ruinous, and others are folded into parks or neighbourhoods, so the best approach is to pick a walkable cluster rather than trying to see everything at once.