Perfectly balanced gameplay.
This game has exactly the right learning curve and lives in just the right intersection between believability, difficulty, and fun. I am not exaggerating when I say this game could have easily been sold on Steam for $10 a copy and people would have bought it in droves, including myself. The author really struck gold with this sadly unexplored facet of the classic zombie apocalypse setting, and it was a game that was just waiting to be made.
I play this game repeatedly, and the more challenging the difficulty level, the more the player is forced to utilize every available resource; from scavenging food, to the desperate race to reinforce the fort, to recruiting survivors to replace those lost in the inevitable fort overruns. My only issues are that the gameplay seems to get slightly stale once your fort grows to a large size, and your population becomes larger. The interface could also be streamlined a bit, but these points are minor. The harder difficulties can get frustrating for me, but that is clearly the point of the game anyway; the potential of losing is always there and there's this great survival of the fittest/no guarantees feeling to it.
The author really should be hired by an actual game company or start her own. Excellent work; the nail was hit squarely on the head with this game.
Perhaps a sequel is in order that could start where the city hall victory left off and you could run a country that rises from the ashes of the zombie apocalypse?