Pay or Grind grind grind

But it is a great grind and kill zombies.

Some of the higher levels are not worth it.
Two things would greatly improve the game.
1: the ability to change guns during a run
( machine gun to mow down the little guys and then bring out the big slow monsters to take down bosses.)
2: the ability to pick upgrades to the guns ( more ammo more ammo more ammo..they never give you enough ammo)

newipadev about Dead Ahead™