Great Game - Minor Tweaks Suggested
I can see where this could quickly become addictive. It's a good game as-is, and with a little more work it could be a classic.
It might be worthwhile to include a note about the recharge rate on the gun, and an indication of where the recharge meter is. It was probably pretty intuitive for most folks, but I sure missed it at first. Caused some amusing framming on the spacebar, though.
You may also consider adding a few more upgrades, or creating a tiered upgrade system if you have time. (Perhaps a recharge rate upgrade for the gun? Or unit upgrades? "Advanced Battle Droids", etc.)