This is a lot of fun, and mind you, I have never, ever said this in a game jam comment - I wish it was a bit harder!
I think you could have a lot of fun with some infinite scaling (adding more enemies and maybe varying them a bit - get some kamikaze tanks, some exploding tanks, some missile tanks), the game is really, really fun!
It is very simple visually, the muzzle flash being 3d is insanely cute, sells the whole aesthetic to me for sure.
Overall, while very simple, this is a lot of fun to play. Controls are very tight and pleasant to use for sure, game makes sense, you understand it immediately.
There is one problem with the game, but it is probably mostly related to the time constrain: if you're on the bottom of the map - everything is cool. If you're on the top of the map, you're blind, since the camera doesn't show you your enemies. I'd rotate the camera with the player (though this is potentially detrimental to the gameplay) or make sure it always sees the whole map by moving it up dynamically - with the camera fixed, it's still a simple, but very much fun game - which is what game jams are about!