Easy.
The was easy at end when you had all upgrades and weapons so it gets booring a while.
The best weapon in the game is minigun one that cost 9000.
Suggestions:
1.Story mode and other people helping and teamwork.
2.Stronger zombies and more diffenrent looking.
3.Boss zombies that are really tough to kill.
4.Zombies attacking human not the base or so the both.
5.Turrets and mines around the base or region.
6.City landscapes would fit more than a desert.
7.When theres night time zombie hordes are more than day.
8.Buying soldiers medics to help in combat.
9.Weapons costs more and ammunation.