As you're so brilliantly smart, you have already deduced Riebeeckkasteel means Riebeeck Castle. This guy Riebeeck was the first governor or whatever (you could not very well call him a stadhouder) of South Africa. He made pots o' money.
Well, we couldn't find the castle anywhere, but there is a graveyard. For some reason, this one excels in growing lichens all over the stones.