I am going to Belgium and I looking for some great castle or chateau to visit. I have seen the Gravensteen in Ghent and going to Het Steen in Antwerp. What other great castle or chateau to see on Belgium?
The castle of the counts in Gent is authentic and attractive, but oriented towards school trips and younger guests. Het Steen is closed to the public in Antwerp, since the Maritime Museum moved out to the MAS. Do consider the Silver Museum in Antwerp for its castle/mansion. It doesn't count as a castle, but you can visit a scrap of Charles V's childhood home in Mechelen. I know you mean a real Chateau, but the Rubens house, Rocoxhuis, Magdenhuis, and Van den Burgh museums in Antwerp are lovely urban mansions.
South of Bruges you can follow the 50km long castleroute with a bunch of neo-style nevertheless lovely castles like Tillegem and Loppem, however Kasteel (van) Wijnendale is original, nice history www.toerismetorhout.be Just a few like the latter two are open to visit. West of Bruges has the Sint-Trudoabdij nice looks, but is not open to visit.
Absolutely nice is south-west of Ghent Kasteel Ooidonk, see: ooidonk.be (with www I get a safety warning, so be carefull too)
Near Antwerp: Kasteel Marnix van Sint-Aldegonde (only by appointment I guess) and Zilvermuseum Sterckshof (as already mentioned) with nearby restaurant Kasteel Rivierenhof for a drink or a meal in a nice setting.
I´m not so familiar with castles in Wallonia, but you can consider Chateau de Beloeil north-west of Mons / Bergen.
Beersel is the only castle that remained as an original medieval stronghold, comparable with Het Steen in Antwerp (I think too still closed) and Gravensteen in Ghent. All others are from later periods or like Wijnendale later turned into luxery residences.
Be aware of something. The Dutch word "kasteel" is a cognate of the English word "castle", but it has a broader meaning than it's English equilvanet. It can refer not just to fortified noble residences, but also structures that in English could be better described as "palace", "mansion" or even "large farm house" in some cases.
Kasteel van Beersel is easy to reach from Brussels by train (the station is right across the street). It's in a partially ruined state.
Kasteel van Horst is a pretty good water castle near Leuven that more or less remains in something close to a late medieval state. Very difficult to reach without a car, though.
I enjoyed visiting the Landcommanderij Alden Biesen near Tongeren, but this is one of those structures where the meaning of "kasteel" gets stretched.
Thanks for all the advises and suggestions.
The town of Ath, between Brussels and Tournai, has a castle. The town square was originally the outer bailey, and I believe all that survives of the castle is the 12th-century keep which you enter on the second-floor level from an exterior staircase.