Solid engine, bland design
The engine is solid, in that I didn't see any bugs, and everything was smooth.
However, you need to put a lot more time into this game. The enemies are all about the same (at least make them LOOK different!). The towers are about the same six towers we've been putting along the sides of roads since the first TD game was made. They all look about the same as one another. Most people will play and love an unoriginal game if it has great art, but there's nothing awe-inspiring about circles and recolors.
This is basically Another Tower Defense Game. Either add some jaw dropping bells and whistles or an original twist or feature.
Also, make it more difficult. I won on my first try with a $12000 surplus and a whole block of level 5 towers that had never fired a shot.