It was built in 1180 and was the castle of the counts of flanders until the XIVth century. Then it became a jail, and then a factory. It was planned to demolish it during the XIXth century but fortunately, and unlike most medieval fortifications in Belgium, it has been saved.
Inside, you can visit the torture room!!!

these were the swords used to behead the nobles who had commited crimes. Common criminals were tortured before being killed.
One torture described was to tie them to their bed and let a plate full of food just out of their reach, then let them starve
Another one was simply to break your bones and let you die, or put you a chain with nails around your neck, so that your throat was pierced by the points of the collar at the slightest move. Prisonners usually died after 3 or 4 hours