They are large but not obviously structured, suitable for smaller people than U to clamber on. Having been moved from the nearby village of Duisburg (not to be confused with the large German city of the same name), it is now still called the Duisburg Dolmen.
An early postcard shows them fenced off from the public, but also declares that they are in fact simply parts of an erratic stone from the glacial period.
I don't have a strong view on that; I will say that initially I was very suspicious about the Belgian habit of moving megaliths from their original locations for the sake of farmers - Irish farmers seem to manage OK - but on reflection I can see that it's nice to put them where everyone can see them, and they occupy a sort of position of honour. (Cf also the menhir at Neerwinden.)