These modest 11th- and 12th-century ruins sit on a small knoll directly above central Valkenburg giving the little town its visual focus, especially when floodlit purple-blue at night. Within the site. there's not a great deal to see other than the view, and much of that you can get for free from the terrace of the restaurant De Haselderhop where you buy the entry tickets (by card only).
To pay in cash you'd need to go around to the Fluweelengrot ticket office (Daalhemerweg 27). There you can also buy a combination ticket which gets you to the castle site via the 'Velvet Cave' system of escape tunnels, but you have to join a guided group and most of these are in Dutch. In December, the caves house a nativity scene and Christmas market.