Really funny & cool idea !
The gameplay is very nervous & unlocking bonuses as you rech highscore is a very neat way to keep the player engaged.
Unfortunatly the basic rectangular map, make it too easy to just back yourself in a corner and start blasting. made it to 68 pretty easily that way.
Not sure how you could prevent that (if you actually went to prevent it). maybe a circular map or a pit near the borders ?
Appart from that, It's very fun & it made me click on retry quite a few time to reach the next upgrade. good work :)