hehe gotta love jimi hendrix.
Good idea for a game. But how creative can one get with a character standing in a stationary position? And the enemies attack the same. How will this game be unique from the other bottleneck shooters? You should check out penguins vs zombies. I think the designer of that game solved the problem of space.
The weapon upgrades look cool. I think you have a great prototype. It just has to be taken to the next level imo.
Here is a game called ghost shooter. It was made with a free program called construct 2. It is similar to tower mayhem gameplay wise.
http://www.scirra.com/arcade/shooter/32/ghost-shooter-demoand here is penguins vs zombies
http://www.moddb.com/games/penguin-vs-zombies