Fun Missile Command-like! It would have been good to have some controls/instructions prompt in-game, though. My first attempt through I only was clicking and didn't understand the cooldown since there was no visual indicator for that, either, but I still survived through wave 3. The difficulty is pretty low overall - it doesn't seem like the player is punished all that much for leaving enemies alive, especially since there's only three waves, and enemies can target cities that are already destroyed.
I think hit effects and sound fx would make a big difference! Having that extra feedback when damaging enemies I think would do a lot to make the action even more satisfying, since getting a good missile hit (especially on multiple enemies) already feels great