So, I got to the end of the Atlantean Stronghold level in TR1 Unfinished Business, but I think I may have 'cheesed' it. Shortly after the start, you go out into a big open cavern, close to some sort of tower. Down from the tower, there is a slanted surface with 4 holes in the top. I dropped down into the lowest of those holes, into a small chamber. That chamber seems to have 3 further holes you can drop down. Two of them are exposed, but if you drop down there, you fall a long way and die on the side of an underground pyramid.

The third hole seems to be behind some screens and it seems like you are not supposed to be able to jump in there. However, I couldn't find any other way out of that chamber, so I kept trying and eventually I managed to (somehow) jump into that third hole, behind the screens. Down there, you fall a long way into a pool; get out, find a switch, press it and the level ends.

I'm guessing I wasn't supposed to do it that way, as it seems the level is way too short? I assume the idea is you are supposed to get to that third hole by jumping into the correct hole in the slanted 'roof' at the top?