The view is kinda difficult (towers are tiny), it's easy to eventually get more money than you can spend (which shouldn't happen in a tower defense game).
Good start, here are some suggestions.
-add sound, sound is very important to games because it drags players further into the game world
-balance seems a little off because enemies take off a lot of the earth's shield (you could lose if you let just two or three slow enemies through) and it's very expensive to increase the earth's shield much
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