A solid start for a full game.
Cool game! This reminds me a lot of "Into the Eagles Nest" on the ol' C=64. Ah, those were the days.
You have obviously put a good chunk of time into this game. The controls are solid and the graphics and sounds are well put together. Overall, I like it a lot. Of course, I have a few complaints / ideas as well.
1. It's only one level. Just when I was really getting into the game, it ends. More levels would be great.
2. Unlimited ammo? Hm. A different difficulty setting with limited ammo would be great. How about being able to pick up more ammo? You could really tweak the levels with a feature like this.
3. The enemies were really dumb. I know how hard it is to program good AI, but perhaps something can be done to make them a little smarter. You can't really sneak up on them much either. A simple line of sight/proximity alert system could be pretty sweet.
Those ideas aside, you have a good game here. I hope you continue to make more!