invisible surface?

I got another problem. I started a small level. I have 2 rooms 1024x1024 one of them is 192 high the other 256. I was made a curved staircase going from the corner of the top room 180 degrees and down to the corner of the bottom room (two corners are opposite sides of the same wall just on different floors). Follow me so far? I wish I knew how to draw you a pic. Anyway In order to subtract space to put the stairs and some headroom I subtracted the actual staircase starting the hightest step @ the ceiling of the top floor so it ran down to the ceiling of the bottom floor(still w/ me?) then to clear the rest of the space in the shape of the curve(of the stairs) I made a hollow cylinder subtracted it away from my level, grabbed half w/ a brush, then de-intersected it. ok? dimensions were perfect. everything is going fine. I drag my new brush over to where I need it(lined up under where i just sub. the staircase shape) and subtract it. Now I have a nice round looking semi-circle carved out to place my stairs in. So I rebuild the staircase brush move it down into position and add it. This is where the probs. start. When I do this it seems @ certain angles in the 3d view half the staircase disappears. I do a rebuild seems to fix all. Ok, add a player start go into level, play. Now everything seems fine but the very top step is not visible. I can walk on it. I can't shoot thru it. I can just see the bottom floor of my level thru it. everything else is normal. Is this because i deintersected my half-cylinder brush in another zone? Is it because the deintersection tool is buggy? Am I just stupid? And most importantly can anyone tell me how to fix it?
If you read this far then, Thx. I appolagize for this being so long I just wanted to make sure you got it. And again I'm sorry for being a newbie when I should've started this 2 years ago w/ the rest of you 8')
smally
gustapo66@hotmail.com

http://24.2.13.222/smally/stairproblem001.unr
hey I tried to make the steps taller/fewer but that didn't work i'm still getting the invisible top stair. I've screwed around w/ it forever. I'm not sure what to do. Have you ever used curved stairs succesfully? Anyway if by chance you wanted to look @ the problem just goto the link above. You can d/l it there. Any other suggestions you may have would help. thx /~unreal/ubb/html/smile.gif You can e-mail me @ gustapo66@hotmail.com

Hey wanderer that fixed it. thanks alot. That order to last thing did it. Although I really had no idea what I was doing and I lost all my details in the top room (I had a different file saved w/ pillars+beams etc.) but that's no big deal. Do you by chance have a link that explains the order thing?