According to their website, the Constantia Nek Restaurant is the oldest restaurant in Cape Town. I think that my mother would contest that as she often feels like her kitchen is the oldest restaurant in Cape Town. ;)
Seriously though, I can't confirm whether or not it's true, but based on the fact that the restaurant is in a spot secluded from the city's development I guess that it's possible that the restaurant is the oldest surviving restaurant in Cape Town. I have to be honest and say that we've never had dinner there, but I do remember stopping by for a couple of beers while watching a friend of mine play guitar in a band.
Back then the inside was decorated old-style, with benches that you'd expect to find in an old English tavern. What was really cool about the place was that they had extra-warm (that's slightly warmer than warm) fires burning inside to keep the place nice and toasty during winter evenings. While I stood outside taking this photo, I could hear voices and laughter emanating from the building - it would seem as though that warmth lives on, fire or no fire.