Cool idea.
I was a bit reluctant to use the rewind button, as it cost quite a bit of power, which meant it felt like I'm wasting money that I could've spent on permanent upgrades (new towers).
The level is also quite small, and the planes and characters moves quite quickly and it's insta-death. The result is that mistakes happen easily (at least for my skill level), and the punishment felt too harsh. Maybe adding lives, increasing the size of the level or reducing enemy speed would have helped?