Alright, I had a quick look at this and I have a few points.
I didn't encounter any bugs, and 100% kills and secrets were obtainable.
However, the map seemed a little bit too plain from an architectural standpoint, especially considering its small size. You could have done a lot more with the starting area before the arena. Since it's not connected to the rest of the level, it gives you an opportunity to create some views that give the map a sense of space. Instead of the simple room, you could have some kind of snowfield with icy cliffs looming above it, and the teleporter could be placed in an abandoned outpost building.
You seem to have made the Mancubis' platforms a little bit irregular as if this will make it look more detailed, but the edges actually look rougher and less attractive than if the platforms were rectangular. Roughening some edges here and there can't disguise the fact that your arena is rectangular (almost identical in shape and size to Doom 2's map 07, actually) Also, that texture you've used on the sides of the platforms looks horrible.
Again, the rough lines aren't particularly pleasing to the eye, but the sidedef texturing is at least nicer here.
This bright red pentagram looks really out of place against the dark brown, especially since its background colour does not match that of the surrounding wall. I also didn't like the jutting bits of wall above and below the switch. Can you not use a slightly more fitting switch texture? I also note it doesn't even vaguely resemble the switch in the middle of the arena.
The walls outside the middle arena are an eyesore. The texture used on them is atrocious, and the walls are totally flat and badly need some ornamentation. The corners are too sharp as well. Please do something with the visuals here.
This frozen waterfall would be a nice touch, except for the fact that it's literally perfectly aligned with the rock face, which looks odd. It would look far better if the waterfall was placed a short distance back from the rock walls. Also, the waterfall cuts off the walls mid texture. The walls are also all too sharp, with not enough curves.
These structures don't seem appropriate to the setting. The first two look like they belong in a demonic cathedral and the third is just placed there for no reason whatsoever and it uses keycard door textures. Retexture it and it could be a nice floodlight.
This secret is wholly unnecessary and annoying, and I think it should be removed. I mean, what's a random stone corridor doing connected to an invisible teleporter in this place?!
This screenshot isn't one of yours (it's from dutch devil's map 11 of the pcorf comm. proj) but I'm posting it because this is how a wall should look.
Finally, try to make the cliffs of the main area higher and add some detail to them, so this doesn't look like a random room that was hewn out of nothing, but looks more like the outdoors plain I assume it's supposed to be.
Gameplay is rather boring and rather easy, but I'm not really sure how to remedy it. Maps with a lot of monsters in a small space can be a tedious exercise in circle-strafing.
In summary, not too bad, but a lot of improvements still to be made. It's currently too much like Dead Simple (which I did not like) with a BFG, more health and more monsters.