What does it actually look like? At first I was imagining some sort of normal citadel, but on a huge scale. But now I'm thinking as a sort of giant, smooth dome of sorts with like openings to shoot from and maybe some large gates/smaller entrances. Does anyone have an accurate image of it in their head?

It's a large fortress made of seamless rock. It's ugly but effective.

This page has a picture of it::

http://encyclopaedia-wot.org/geography/tear/tear/stone.html

Here is the picture itself::

 

I've always been under the impression that it slightly resembled Ayers Rock, only larger, hollow, and much more rounded.

See, that's what I thought, a large dome of sorts that was used as a fortress rather than something that resembled actual fortresses.