Nice game
A few suggestions though, if there is a sequal.
It was far too easy. Here's a few things that could be done about this.
1. Make the weapons weaker! You can survive the game with the first pistol if you have the patience. With your other survivors (especially the one with the rifle) you can keep the zombies off your barricade easily. Also, if you made it so weapons didn't have an infinite amount of ammo (except for maybe the first pistol, since you own it), it would add to the difficulty. You wouldn't find endless stocks of ammunition if you killed a zombie that was dragging a rifle along on it's back.
2. Zombies were too easy to kill! Make some more zombies with various weak points. Soldiers, while a little harder to kill than other zombies, were still able to be shot in the face and legs. Maybe make some soldiers with kevlar on thier legs too, so you can only shoot them in the face, maybe make some zombies with only leg shots, others with only body shots.
3. Another fun feature would be health bars for each survivor. This would allow for things like dogs to jump the barricade (rather than standing at it, because a zombie dog wouldn't attack boards and barbed wire, they would try to jump over them) and attack the survivors directly. Have one survivor be trained in first aid, so that once you find him, he can heal the other survivors.
4. It's too short. Lots more levels would add to the fun of mowing down these zombies.
5. How about some more variation in the weapons? No single weapon was vital in surviving this game. Any one weapon could get you through all twenty levels, as long as you had survivors to back you up. Perhaps some new weapons could be added; some of them could be weaker, but have a much faster reload time, others could do monstrous amounts of damage, but have an equally monstrous amount of reload time. There could also be some more unconventional or exotic weapons. Molotov Cocktails would be readily available to a group of people out in the woods, whether they were filled with gas (which would be around for fueling the chainsaw) or hard liquers. Blunt weapons, like baseball bats and edged weapons like machettes or katanas (found in some eccentric collector's house) would also be fun.
6. Maybe you could add a random day-time attack feature. Zombies wouldn't wait around all day only to attack at night, so why not have your survivors suddenly have to defend themselves for a short period of time during the day. If some are outside of the barricades, have the ones still inside hold off the zeds long enough for the ones outside to jump over the wall.
All in all, a great game. I hope there's a sequal. I give you a 9 in graphics, because it looked great. 10 in style because I love zombie-defense games. 8 in sound, I liked the zombie groans and hisses. 10 in violence, because what zombie game that's worth playing doesn't just overflow with gore and violence? Interactivity got an 8, maybe if you could arm your survivors and tell them where to hold their ground, it would add a more controlling aspect. Humor got a 7, because of some of the zombies. I loved the fat ladies with their hair all poofed out.
Overall, I give you a solid 9.
(sorry for all the writing, but I love zombie games and if I could create games, I would love to have all these features)