The up and down keys should be just that, up and down. It would also make sense for the left and right keys to control the plane's speed. I'm not sure if this dampens the playability any but it's worth noting.
Pros: Huge upgrade system.
Cons: The encyclopedia needs a definitive stats chart for enemies and towers. (Opponent's strengths and weakness per tower.)
 "Bomber At War"){kind=icon}
 "Kingdom Rush"){kind=icon}