At the Steen castle in Antwerp. Imposing architecture when admired from up close with some good architectural details. It has a fascinating history, and today it is one of Antwerp's oldest buildings. Not too big, but a very ancient fortress. It used to be a museum inside, but it was moved elsewhere, so it's a kind of children's play area now. Recommended, but just to see this I would not recommend taking an enormous detour.