I just wanted to post here after stumbling across this forum. I have had anxiety issues on and off over the years, but nothing of significance for almost 10 years.
I have casadastraphobia and have for as long as I can remember (4 years old is my earliest recollection). When I do look a many of the scenes mentioned by other posters, I lose all feeling in my legs from the knees down. This means that I can't feel the ground so I feel like I'm lifting off the ground (illogical, but hey, phobias are). I've always described it as a fear of plummeting upwards, or a fear of infinite depth.
Phobias aren't always a fear sensation, it can also be an "irrational aversion", and that's what I experience. The idea that it's some form of balance problem or vertigo makes sense. It generally only happens when my head is tipped quite far back. Then with each second and I get more and more convinced that I'm going to "plummet up" and eventually I'm compelled to stop looking. Once I straighten up I feel a little fuzzy for a few minutes, but then I'm fine. No fear or anxiety, just an intense and irrational unpleasant sensation. It got bad when my anxiety was at it's worst, but these days I find it amusing and I experiment with the sensation to work out what can set it off.
I guess I just wanted to share that to be reassuring. Chances are casadastraphobia is a balance issue that exists on it's own, but when we have anxiety problems at the same time it gets exacerbated. If you find a way to manage or treat your other anxiety issues, this physiological issue can just be an odd quirk that you forget you even have. That's my experience and I wish you all as much fortune, i.e. successful anxiety treatment/management and quirks that amuse oneself instead of stressing oneself!