A must see when visiting Ghent. Also a great place to explore with children. Lots of impressive rooms, thick walls and stairs. Be warned though, there are some rooms containing disturbing images about torture and executions.

We visited the castle with two children of four and five years old. They both absolutely loved it.

Gravensteen Castle, Sint-Veerleplein 11, Ghent 9000, Belgium
https://gravensteen.stad.gent/
+32 9 225 93 06

Visited June 2015. You can find my review about Gravensteen Castle also on TripAdvisor.