Great job, improve for sequel.
First things first, this was a great game.
Now here is how to improve it.
Defence needs to end at some point.
Instead of having different kinds of the same type of weapon, have on of each, and make it upgrade able with different upgrades. Such as more damage, larger clip, less reload. That makes the weapons more of a personal preference and much more fun.
I would like to see some big ass zombies.
Definitly would like some boss levels.
More sound effect.
The zombies death animation should be slightly different depending on weapon.
Ability to put turrets on the base would be nice.
When you upgrade your base, it should change the look, you should be able to upgrade armour, size, upgrade the fence, electrify the fence. Stuff like that.
Your guy should be upgradeable too, you should be able to buy helmets and different types of body armour. A laser sight for your helmet. When you buy a helmet you could add upgrades to it, like a hud with like radar and advanced warning systems and such.
A rage mode would be nice, you reach a certian amount of exp or something or you fill up a meter and everything goes into bullet time mode.
ability to upgrade the weapons should include stuff like laser sights and extended/sawed off barrels, scopes and so on.
Objects would be nice, like exploding barrels, and placeable walls, mines and thumpers would be nice too.
Also the ability to release small animals as distractions would be killer funny. like release a bunny and the zombies all start chasing the bunny. =)
Consider these ideas for a sequel. =)