Nice game
After few hours of trying, I finished Quartermaster in 72 minutes ;D
Really hard game, though save option helps greatly (it would be unplayable without it, one little error and you are done).
Like many others already said, there should be wider range of weapons - maybe homemade molotov cocktail, or simple mines, etc. Reinforcing towers or placing barbed wire on the ground (since zombies are dumb, they would go straight into it, slowing themselves greatly).
When I was on my way to 3rd bridge, I've first cleared zombies in the area, fought through another wave that luckily came from side of main base, and tried to lead people to TNT using only depots. But it wasn't that easy - they just didn't listen at all. Solution was a bit weird though. I was raising number of people needed in one depot, then in next, and so on, until TNT.
Game's really good, for a flash RTS it's one of best I've played. Specific mechanic of people movement was annoying at first, but later it was fairly easy to steer them out of trouble. Still, destroying depot in the middle of chain, then destroying one with people, makes them stand in one spot until another depot is built, or zombie is close by. Make them always go to closest depot, and it will be ok ;P
If you ever release 2nd one, try to make wider selection of towers, also - sniper rifles are rare comparing to pistols, submachine guns or shotguns, so keep that in mind ;)
Well, in general, great game, towers aren't overpowered (beside a row of 3 snipers with 11 people + 2 upgrades on each), there's always need for one of resources (especially ammo, and later - people), and player is forced to think a lot.