Riber Hill is the place where Riber Castle was built – I remember going there as a kid, it had a very poorly looked after “zoo” type thing. It wasn’t built until later than Rhiannon’s source, in 1862, so may have been built over the top of the Druid’s Altar (or even used the stone!).
Here’s a bit about the castle (scroll down the page to “Riber”):
Ha interesting detective work, tsc. That's a shame though if they did stick a castle on it! but I expect they wanted the benefit of the view that the Druidic Altar (whatever it was) would have had.
Having looked further, the zoo went bust a few years ago and the castle is now a hotel, so perhaps TMAers can spend the weekend at the spot!
As I look out my front room window the castle is still a derelict shell....the hotel/executive apartments ideas get floated every once in a while but never get further than an artists impression in a pamphlet. Last time anything happened up there was a coupla years ago when the Sheffield Chapter of the hells angels rented it for a party one weekend.