Board Thread:Questions and Answers/@comment-33636382-20171113044643/@comment-26484417-20171113125053

Well, it's easy to imagine that the Tower has physical wall that surrounds it's outer shape, so let's do just that.

Now, we know that one of the strongest beings in the Tower, Urek, has tried anything in his power to leave the Tower. We know that he isn't shy to use brute force to solve his problems, and yet he is still trapped inside. This leaves essentially two explanations:  The walls are indestructible, or at least require more power than Urek can muster in order to be destroyed. I imagine that sufficiently powerful Axis could still do it, though. The walls are breakable, but such an act threatens the lives of the people inside of the Tower (e.g., the entire Tower collapses), threatens the lives of the people in the outer world (e.g., if Shinsoo replaces air, most people outside might just suffocate), or both. None of which sounds like acts which Urek--or other similarly powerful individuals--would desire.  In conclusion, the walls "cannot be destroyed" because it's either not possible or because the side-effects would be beyond undesirable.

PS: Naturally, it could also be that there are no walls as we imagine them, in which case it could be that the space loops onto itself where one would expect a wall. In this case there is nothing to destroy in the first place, so that means of escape is unavailable.