You should label what the different towers are and what they do. The really tall tower shouldn't just target the enemy at the front of the line, but the one closest to it. Maybe even add in the ability to tweak the targets for the towers, so I could make one always shoot at the weakest enemy, while another shoots at the one closest to it.
The final level seemed to start really quickly, maybe give the player a bit more time before spawning so many enemies.