Great game! Addicting!
A few tips to survive in nightmare mode:
Build up a defense:
Go to a side of the map, where the monsters don't spawn. Build a barricade about 3 layers big (at least), then build turrets behind it (mortar turrets are most useful). Make sure you space your turrets out into columns, so that you can walk between them and can easily replace the ones that run out of ammo. The best spot to build a defense is usually along a canyon wall of some sort.
Bomberman Style:
A neat trick you can do to go out and collect the resources is to go (what I call) bomberman style. Put a line of barrels from the front to the exit/entrance of your safe zone. When your barrels explode, make sure you have barrels equipped and keep hitting the space bar. You'll notice that the explosions will keep your barrels that you lay down exploded. So, you can run into hordes of monsters, blowing them all up and be relatively safe.
Just before you run out of barrels, make a "B-Line" back to your base, to avoid being chased by zombies. If you're fast enough, you can also lay an air-strike down, right when you run out of barrels. The monsters will be covering the airstrike beacon by then, so they'll all be blown to smitherines, when the bomb is dropped. :D
The longer you can maintain your defense area, the more points you'll get and you'll see yourself shooting into the high score area. Good luck! :D
10/10