This game is a great arcade-esque style shoot em' up and I adore it. I played for a few hours with a friend and it was a lot of fun but I have a few suggestions to improve gameplay:
-You can put a point after every round into a skill
-Skills could be like: accuracy, damage, run speed. and scavenger (get more from cash/supply drops)
2. GUN UPGRADES
-Pay to get various upgrades/attatchments to your guns
-Such as: incendiary (does dmg over time) explosive (hurts more than one target) or like an increase in damage, accuracy or firing speed
-We need just a little bit of character development, who is "The Girl"? Or "The SWAT"?
-I suggest adding MORE characters and giving them each starting attributes depending on who they are like: The SWAT gets a defense bonus due to his bulletproof vest.
-You should add more traps and trap like elements to the game
-Such as a bear trap that costs, say, $50 and you could put them in a 1/4 space of a normal tile
-Fire traps would be cool
-Palisades should decay with damage and you should have to pay to repair them
-The advantage of this would be you could add in palisade damage. As in they do damage to zombies based on how new and freshly repaired they are.
These are just a few suggestions that I thought could make this game better, even though it already is pretty addicting.
P.S. Add some detail to the map, just a few cracks in the concrete or litter. Something to make it more immersive.