Extraordinary Game!
I thought this game was superb to most 'zombie defense' games which all more or less are replicas of each other.
So this was by far refreshing game to play and very addictive too.
I'm going to vouch with everyone else who has posted there should be a sequal, because there should be.
Now as everyone wants to be a game designer I have a few suggestions:
1.) More survivors, maybe ten including yourself
2.) More levels, maybe 28 days then you can parody the movie
3.) More weapons, perhaps an M60 or M16, rocket launcher, minigun, etc.
4.) Better interactivity, I thought the game play was somewhat choppy (it could also be my computer) and maybe proficencies so you can increase either your accuracy or damage
5.) More defenses, maybe a tower for a 'sniper' survivor
Those are all I can think of right now, but overall I enjoyed this game very much and I'm glad you shared it with us.
You have real talent and potential.
Keep up the good work!
~Alecterum