Good system, but
the "primary and secondary weapon" thing is completely pointless. Your weapon set doesn't cooldown while on the other set. There's no real benefit to using the other set. Really makes the game unenjoyable (especially when you get later in the game when you NEED that first set cooling while using the second set).
Also, a sandbox mode would be cool for this (i.e. if you die, you get to try that level again so you keep your upgrades).
Castle Crashers!
Dude. That Main Theme Song... that's from Castle Crashers!! Awesome =P
...But average game.
Mildly entertaining
It wasn't very much fun, and it got repetitive. Age of war is much more fun.
Some suggestions
Fix the lag, I can not stress this enough, even when i use the lowest graphic setting I'm getting lag when the enemies start to swarm.
Another option is to let people go back and play previous levels or allow us to recycle and reassign our points if we find ourselves getting stuck. There's a few times I've wished I could take back upgrades from one thing, and dump them into something else. Last, and endless mode would be nice to maximize the chances for players to get everything after they beat the main campaign. The game is fun, but stuff like lag and the difficulty balance need work.
All in all...
I found this game was not bad, but at the same was not very good. The guns seem very underpowered and make no sense. Why does a rocket shoot like a shotgun? Another thing I disliked is the fact the soldiers shoot from the very back of the stage. The special weapons were of no use in the later levels and the spell is a pain to use. I would suggest you make the guns and grenades better as well as move the soldiers up more before they attack.