A very good first attempt
The concept is great; very fun and easily addictive.
Graphics are good for a game of this type. Controls are smooth, the interface is easy to navigate, things aren't too confusing.
The main problem I had was the scaling of difficulty. The game is just too easy, even on the hardest difficulty setting. I never once got killed or anywhere near it. In addition, your health is fully replenished at the end of each round. There could be a little more variety with weapons and/or zombies to fight.
I wish there was a little more to do with money. It's not too hard to gather, especially if you stumble upon a double cash upgrade. 30 seconds of double cash will get you all you need in one wave. The barricades also didn't seem significant enough. Sure, they kept the zombies out, but with the difficulty the way it is, I never saw the need to go out of my way to repair them. Sure, if I was passing by I might hit F and keep on moving, but I played most of the game in the southwest corner of the area: ample room to run in circles and no fear of getting caught.
My main suggestions:
1. Increase the difficulty - the game is just too easy (harder to earn money, zombies do more damage, health doesn't regenerate at the end of the round (perhaps pay money to get health packs?), barricades help a little but don't seem necessary)
2. Add more variety - more weapons, zombies, etc.; whatever you see fit
3. Add more uses for money/make money harder to obtain
Aside from that, this gave was extremely well made and very fun to play. It kept my attention for a good 20 minutes before I got bored. Really, great job!