to be honest, not one of the best break-out style games i have played. level 3 is daggon near impossible. as the others have said though, make the powerups bigger and slow the ball down a little.
it can only get better though--
like everyone else said, the game is simple if you use your maths skills. i like this game though...gives people good practice i guess. keep at it and try to implement three and four variables. keep it up.
there is a glitch where you can have negative money.
if you click a tower to buy, then click a tower you want to upgrade and upgrade that tower, you can place the tower you were originally going to purchase...ending up with negative money.
from what i see so far, this is a good game. excellent concept- fix some things though.
as others have said, just gray the tiles out and add a send wave button
find a use for money upgrades?
make it so you cant place towers on top of one another