Ipowell, I'm Going To Give Some Constructive Feedback.
So I'm pretty late to the party with this one, but I thought I would give my 2 cents anyway.
I really think you should take some time and flesh out the "feel" of the rooms a bit more. This gave me a vibe of the stuff that was coming out in about 97 / 98. It's by no means awful, it is just "simpler" than what we're accustomed to.
Off the bat there are two things that I think would help this map greatly. One is rock terrain, have a look at socks articles (http://www.simonoc.com/pages/articles.htm) on rock walls, I have found them invaluable.
The other thing to think about it lighting and maybe even fog. I know that fog requires a modern engine, but it allows for atmosphere in "void" areas.
Alternatively hard straight edges on void areas also seem strange. Here is some "done" well. http://spyhunter007.com/Images/american_mcgees_alice_bottomless_void.jpg
Here is your area, http://imgur.com/L09LUh2
Note there is no transition into solid geometry... The solid geometry just starts abruptly.
You blended the void in quite well in the areas with the blue bricks and switch, you didn't follow it through to the rest of the map, I felt this was a shame.
Regarding lighting, you could probably drop the brightness quite a bit, particularly in the earlier areas, this would add to the atmosphere somewhat.
In terms of gameplay, I thought you did quite well. The encounters were well balanced, I was playing on normal and didn't die to any enemies. The open areas in the "end" encounter allow the player to run away and snipe things from a distance. I sort of cheesed the area. I'm not sure whether this was by design or a coincidence of your choice of geometry. If you were to shrink this last area I'm sure it would be a little tougher for players.
All that aside, congrats on your first release. Well done :)