A feature of an excellent tower defense is the ability to set the towers to target specific types. Highest Hp, Lowest Hp, Fastest, Slowest, Ninja, swordsman etc etc. It adds an entire new dimension to these kind of games. Also perhaps giving certain types of tower a slight crit against certain types of enemy.
There has been far too many of these kind of games lately. Puzzle solving is fun, but throw something else in just so I don't kill myself out of boredom during it.
This is an awesome game. The awards are so cool. Lots of different enemies. With the rewards you can choose how you want to play to a large extent. Decent music and a mode called "hardcore" I think this is the coolest game on kongregate. 5/5
I don't think a "shop" would be a good idea...