Really great!
For a flash game this is probably the best i've ever seen.
This game is a great defence game, allowing people to build up thier barricade, find weapons or survivors means you gotta mix things up a little, however, it was almost too easy, once you had the Uzi anything could be beaten (a combo of Uzi and Shotgun meant that barely any zombies ever touched my wall).
I think you could make it harder, add a 'food' element, finding food takes time, so if you don't have enough food, you won't fight as well as you could (aim becomes sluggish, other survivors will retreat).
Allowing the fort to be built up more would be good, make zombies harder but allow things like reinforced walls, barbed wire, and maybe even gun towers, so we end up with heaps of hard-ass zombies but then you can retaliate with a couple of MG's.
But other than it being a little too easy and repeditive, it was good (maybe make it that after a while, all you get is fast zombies and SWAT guys.)