Interesting idea and execution!
I'd appreciate a timer telling me how much time I have left to survive. When I survive the whole 30 mins, it also says "Game Over". This could because it's a demo, but nothing unlocked and the game didn't acknowledge it so I didn't know if I did something wrong.
If you cycle through the same four weapon upgrades until they're all Level 10, the remaining three can only get to Level 1 before they're locked permanently. Is this intended?
Also, you need to explicitly state the limits of the demo. I don't know whether level 2 (mushroom) can be unlocked. If not, what's the difference between that and the greyed-out levels?