The ominous music and the design of both the background and the sprite are really nice visually, and the animation as well as the sound effects are a really nice feature that fit well with the theme. My biggest issue is the ambiguity of what some of the upgrades actually do, for example when purchasing the zombie I didn't notice any changes to the click power or points per second, so some info on each power up and ability would be a good addition including how much each item costs.
Other than that and some balancing in regards to certain upgrades this is a well made game and wass fun to play.