This massive square flat-buttressed keep was built by the Count of Hainaut, Baudoin IV the Builder, in 1166, to defend Hainaut's northern border with Flanders and keep an eye on local nobility. This military complex has a four-storey interior. Note the lack of windows in the lower section and imposing fireplace. The keep was used as a prison in the 14C. The enclosure added in the 15C and 16C has been converted into an arts centre.