"Oh, that's simple." Not really, but it can be simplified. "This building, or something very much like it, is a constant across a number of universes. That gives it... Hmm, something like gravity, I suppose, is a good analogy. Travelling between universes isn't a terribly precise business, and so something like this makes for an excellent anchor point, if something goes wrong. Which it must have, if I got pulled out of my travelling spell, with this universe the closest to where I was at that moment."

He pauses for a moment, and licks his lips.

"Or someone did something recently to disrupt time and space around this particular iteration of this tower, in which case I probably should have used a different anchor."