From sped-up zombies to restricting use of specific turrets, Challenge mode presents cool scenarios that take too long to crescendo. The path to each challenge's noted score objective is a long one. Shorter stages based on tough waves of zombies instead of a general score would give Challenges better structure and greater appeal. Right now, they feel as if they go on and on and on.
Persevere, though, and you're sure to find Zombie Attack! an amusing experience. Creative turrets and satisfying upgrades make it fun cutting up the undead masses, even if it the gameplay moves hardly faster than them.