I think the game is pretty fun, but there are a handful of minor annoyances that keep it from shining. For starters, it seems like the text is a little too small. Maybe it's from playing it in fullscreen on a high resolution monitor, but you should definitely have it scale if that's the case. Without the text being a meaningful size, the health/score mechanic is a little hard to visualize. It might help to have some graphical component tied to this to make it feel more obvious to the player. This also seemed to affect the upgrades. I had to be looking at a very specific part of the screen to tell if I got an upgrade. You should get a very encouraging/obvious sound and couple it with a visual flourish (either particles or maybe an image with a tooltip telling you how to use it).
The ammo system needs some sort of visual indicator as well. It's hard to tell how much ammo you have left and when it's reloaded. Some simple numbers or visual ticks around the tank would go a long way here.
Finally, the sound effects aren't bad, but they're loud and a tad repetitive. Maybe tone down the volume and modulate them a bit so they don't sound so similar every time. I think with a few tweaks, you could have a really fun, Missile Command-y game here!