The difficulty steadily increases throughout the single player Adventure mode, and during it you'll eventually unlock 48 different types of plants to use against 26 different types of zombies. (xblafans.com)
The single player adventure mode is broken into four parts, and if you happen to die it's frustrating as you'll be put right back to the start of the game. (mkaugaming.com)
After all, it's a single player adventure game. (ab-gaming.com)