I'd say arctic, but with the classic tf2 spytech twist to it, featuring a front area that looks like a normal building, the initial interior looking normal, but all of it is concealing a hidden high tech base!
The bases in Well are actually bigger on the inside; they accomplished it by hiding the biggest room behind the skybox brushes and making sure there are enough twists in the hallways that it never renders from outside.
Big Foggy map where ya can't see anything near ya, and ya have to guess where the enemies are at! :T