The Dark Ages is the next world in the game. You'll find the previously unlocked grave buster useful again in these levels. Straight away you'll be introduced to the useful sun-shroom.
Here is a complete rundown of all the new plants introduced in this world.
Sun-shroom - Level 2
This mighty plant starts small and costs little but over time it grows bigger and will generate more sun according to it's size. I love this plant and always choose it as it is so cheap to plant a lot, and it's not very long til they are deliver a whole heap of sun.
Fume-shroom - Level 5
This plant is a pretty decent alternative to some long range plants, it shoots out damaging fumes which hits multiple zombies that are closeby in the same lane.
Sun bean - Level 7
This is a sun generating bean that also does damage to zombies. Pretty useful and interesting.
Magnet-shroom - Level 16
This plant will strip zombies off their metal apparel such as helmets thus lowering their defense.