First of all, this is one of the greatest flash games I've seen. It's a building and defending game at the same time, very well balanced, with a couple of buildings and events, and with loads of possibilities. I agree, it's most fun to play it in "hard" mode.
I like it as it is - but as I've already played it sometimes at other places, I have some suggestions to make it even better...
1. It's a bit cumbersome to get your survivors off and on guard duty every move. Perhaps it's better to automatically assign every survivor who's not on a mission to the fort. As you can't protect survivors from being killed during attacks by not sending them to the guard duty it seems pretty pointless to have idle survivors at the end of the day.
2. To select more than 10 soldiers for a mission you have point, click and draw a lot. Perhaps you could make the survivors in the list a bit smaller, put them in two columns, or something similar.
3. After you closed the portal the zombies pass away by and by, and no more zombies come around. The rest of the game becomes easy - too easy, I think, as you only need to first kill and then reclaim every remaining tile.
Anyway, great game. I'll give it another try right now.