I like the initial idea, but there are a few problems. I like the fact that every tower has health and that enemies can damage them, this makes it so that i always have to watch the screen if a tower dies making it an interesting bullet hell. On the final level the bullet hell aspect gets out of control, with tanks and planes constantly killing your stuff. Another thing i like is that the best tower is the one with the lowest health, giving you an incentive to purchase lower-tier towers because of their higher health to tank for your better towers. However, this also makes it frustrating as the planes constantly damage everything and increase in numbers as the game progresses, especially on the final level. It just makes you lose without feeling like it's your fault because there is no way to counter the planes. Maybe add a tower that shoots the planes so that my towers don't die in 2 seconds. Another bad thing is the targeting, half of the time they shoot the first enemy, other it's either the last or the strongest enemy. Also, the main reason the game has difficulty is because of the planes, it doesn't take a lot of brain power to spam red and green towers in the beginning to counter the tanks. And the missile tower just destroys everything.
My suggestions
-make it so that planes have health and add a tower that only attacks planes
-fix the targetting so that it's predictable
-Reduce the number of planes but increase the number of enemies, their health and reduce the money they give by a bit.
-Add a difficulty choosing a system. -Make it so that when i place a tower i can see the range of it -Increase the price of the missile turret so that the other missile turret doesn't get overshadowed -Maybe add towers with effects like slowing or fire. But im afraid the game might get bloated if you do that
The game right now is just, place green and red turrets, wait for moni, spam missiles and red towers, and hope that they don't get destroyed in 2 seconds by the insane number of planes.