I like the concept - the way the one level expands and you collect more weapons is really neat and has an interesting way of pressuring you. Certain choke points become advantageous to hold, but because of the missions and doors opening, things stay dynamic and you run around the level quite a lot, especially once you start buying health, ammo, and extra lives.
Negatives? There should be at least another enemy type (flying would be good as it would require the player to aim in more directions.) It's also too easy to get big chunks of money at a time and skip to the best weapon of each type, or just rely on the mystery boxes, whose weapons are powerful even before you buy any of them. The missions are few and far between, so much of the game feels very repetitive. With the amount of guns you can buy quickly superseding the amount of content, there's not much reason to unlock them.
The one level gets kind of old after a while, too. You did a great job designing it and it feels pretty balanced, but there's only so long you can play it before it starts to get stale. It would be cool if there were some areas that cost a ridiculous amount of money to unlock (like $1000.) That way the one level could retain some variety even late into the game. Time bonuses on the missions would also give the player a sense of urgency.
I've noticed a few bugs. You can stand on the leftmost obelisk when the mission comes up and enemies are unable to reach you. Sometimes when teleporting to the top of the level, enemies get stuck inside the floor, and when you restart a game, sometimes your Beretta will start with the amount of ammo as the last gun you equipped in your previous game (one time I actually spawned in an inaccessible area - no idea how that happened.)