This is a really great game! its got some really surprising replay-ability. the first game, i didn't realize the upgrades stacked, so my next game was very different. another contributing factor to this replayability is the random spawns of each upgrade (although, i think there's a bug with the 'reload speed' spawning, i never saw it).
I could totally see this being a full mobile or flash game if you added a few maps, different modes, and cleaned up some of the animations/graphics. Awesome job!
Edit: oh yeah i forgot, there really needs to be an indicator of the tower's range (other than the number) and some way to tell what level each plane is.