Great Game
This game was awsome man and I have a few suggestions in case you decide to make a sequel.
1. Traps, perhaps a draw bridge that is controlled by a switch or something like that.
2. Destructable environments, although that would be a bitch to program and would probably cause glitches that would be pretty sweet.
3. Ammo limits, if there was a limited amount of ammo and ammunition pickups scattered through-out the levels it would be more challengeing.
I am aware that all of those things would make that time that it takes to make the game alot longer but those things would kick ass