Doorwerth Castle
Wageningen is in the province of Gelderland, and Gelderland has a remarkable selection of castles. One of the most impressive is the medieval Doorwerth castle, which is (not surprisingly) near the village of Doorwerth and within cycling distance of Wageningen. The other good thing about that castle is that it is right next to the rather steep edge of a glacial moraine, which is quite challenging to cycle up and down. It is not that it is so super-steep (about 15% maximum, but mostly 8-10%), but that the steep bits also have loose sand and roots across the path in such a way as to make life quite difficult. The path nearest to the castle is sufficiently difficult that I always feel quite a sense of achievement getting to the top (even if I have to stop and reposition the bike a couple of times) and if the sand is really dry, in the summer, it is (at least for me) quite impossible.
You can find a couple of routes to cycle there and back from Wageningen here.
