Addictive and Fun!
This is an overall good defense game. It's fun, interesting, unique, and original. Not too many good ones like this.
The only complaint I have, is similar to many other reviewers. The cannons end up costing so much, it gets almost impossible to build new ones. Even the upgrades for the cannons are insanely expensive.
Aside for that, it's great. Nice and challanging, addictive, and overall, damn fun.