Cute art and nice color scheme.
It was nice to have a wizard to control, but it didn't look like I could actually do anything with him. Does he attack?
Also, I would have wanted to have some information about the different towers, including their cost. Without that, it's hard to choose what to build.
Finally, it's good to add a little bit of music and sound effects. It adds an extra dimension to a game, even if you don't spend too much time on it.