Koth_?bunker?
MID is a bunker, the only ways to enter it are two tonnels and a big crack in the ceiling of the bunker. You can get on the roof of the bunker by going up the stairs inside the tonnel or climbing the ladder outside the bunker. There are also two small towers which are great sniper spots accesable by climbing a ladder or throug the second tonnel. The second tonnel is connected with bunker tonnel, but there's a cliff, so it's kinda dangerous out there.
All of this is located on an island. The tonnels go throug 2 giant mountains, so the bunker is between. The spawnrooms are smaller mountains, more like hills.
That is quite difficult to understand, so here's my crappy scheme.
And 3D model. You know, maybe you could check it, and if you will like it, you could make it in the HAMMER?

3D Model: https://sketchfab.com/models/910fc171cb2740b7b7c7e5d6329aa54e
crappy scheme: http://imgur.com/gallery/ICs5F