I’ve done it twice – in London, Up the O2; and on the roof of Uppsala Castle. I believe these days you can find something like it in many cities. I definitely know about one in Stockholm 🙂
It’s quite fun, also a good test of nerves for those who are, like me, not especially good with heights 🙂 And the view is always amazing, which is, most of the times, the point!
You put a helmet and a harness on, get attached to some mechanism and walk along the designated path. Absolutely safe yet still absolutely scary 🙂
Look for it, you might have it in your city 🙂 It’s a nice little adventure!