After a short while into the game, you can stop the monsters from respawning, and when you do decided to make them respawn they'll be harder, and drop orbs required for unlocking features on the respawn towers.
Its a really fun game, plus you can run past a huge majority of the enemies as you get stronger.
Looks like a nice game graphically. Glad to hear that the respawning zombies can be stopped, coz that's a definite turn-off for me. Let's hope the bosses aren't too hard...coz I'm a wuss.....I prefer to enjoy my games, rather than get my ass kicked repeatedly.