There are actually an awful lot of round doors, windows and other portals in X2. At the bottom of this post are a series of screengrabs I just made to demonstrate this point.
Nonetheless, I think you'll see just which one in particular it is that Bryan Singer is having rebuilt for Days of Future Past. Here's a pic he posted to Twitter:
The accompanying tweet was a nice tease too:How many days to Comic Con?One hundred and forty... seven, I think? If we're counting until preview night.
Now, here are plenty more pictures of round things that you can walk or look through, ala Singer and Guy Hendrix Dyas, his production designer on X2.
Motif, OCD or whimsy - you decide.