I want to be able to see the mini tower health as well, the visual changes alone aren't enough to tell whether they're 1 or 5 shots away from dying...
Also maybe you should make the fact that you hit the enemy more apparent, like making them flash red? Because it just looks like they're dodging it when they're actually being hit.
Maybe you can make the upgrades, manager and achievement screens cover the whole screen? It's too small occupying just like 1/5 of the screen, and the pros need to rapidfire buy upgrades, so instead of scrolling make the upgrade screens bigger.
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