Diekirch is a compact town on the Sauer in northern Luxembourg, useful as a transport stop for Vianden and better than that sounds. It has a walkable centre, a riverside park, a cluster of museums, and enough old-town texture to reward a slow afternoon.
The town's strongest sight is the National Museum of Military History, one of Luxembourg's major Second World War museums. Nearby, the Museum of the History of Diekirch digs further back into local Roman and medieval layers, while Saint Laurent keeps the old centre anchored.
Diekirch also has a lighter side. The town is associated with beer, donkeys, carnival, and summer fairs, which is why the Donkey Fountain feels less random than it first appears. If you are passing through on the way to Vianden, do not just change buses and leave.
Most travellers will see Diekirch as a half-day stop. Stay longer if you are interested in Battle of the Bulge history or want a quieter base in the Nordstad area.