Too easy.
It's just too easy. It's boring. I don't even have to think for... well, any of the levels. I don't think this is a problem with the level design, I think it's a problem with the concept. I can't imagine how you could make a level for this that would be very difficult.
Some of the levels with simpler geometry were interesting, eg levels 27 and 28. I think it's because the simpler shapes make it into a more precise, logical puzzle than a fuzzy visual-thinking one. You can just look at the topology of the level in order to work out where to place your towers to get 100ยจ% coverage, rather than just sort of fudging your placements next to corners and stuff in order to increase your coverage as much as possible.
So I would either give up on this particular concept, or reduce it to something with simpler, more precise rules. Notice how a lot of puzzle games take place on a grid (tetris, meteos, polarium). This is because it lends itself to simpler rules that can be parsed in purely logical terms by the mind, which is usually more satisfying than just sort of guessing and adjusting and dealing with nitty-gritty little visual details.
In any case, for future puzzle game ventures, remember to use better music (the music is nice, but the loop is too short. Ideally the music in a puzzle game should be nice enough and long enough to listen to on its own) and maybe avoid having ads on the edge of the screen. That's all just part of making the game relaxing and appealing to the eye and ear, which makes for a much more pleasant environment to do puzzles in. Also include a better interface (for instance, why is there no way of previewing a level from the main menu? why can't I click "next" on a level until I've beaten it? Why is there no way to go back to the previous level? etc).
But, nice effort.