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Plants Versus Zombies is all about keeping zombies from getting to your home, and the Chomper Wall defense is an excellent strategy for keeping them at bay. It also is relatively inexpensive and very effective against most types of zombies. You will need Peashooters, Chompers, and Wall-nuts for this defense.
The concept is simple. As resources come in, focus on building a line of Wall-nuts across the map about three spaces away from the right side of the board. This is easy to do thanks to the low cost of that plant. Before the first wave of zombies approach the main form of offense against the zombies will bit the basic Peashooter, which is more than adequate for fending off zombies pre-occupied by the Wall-nuts. Place one Peashooter in each lane.
Once the Wall-nuts and Peashooters are built, you'll probably have hit the first zombie wave of the level. This means it is time to start placing down Chompers. Place each Chomper directly behind a Wall-nut. The Chomper can eat any zombie one tile before it, but it has to take some time chewing the zombie before it can eat another. That is what the Wall-nut is for. The Wall-nut will keep zombies at bay, allowing the Chomper time to digest before its next feeding. Once everything is complete, the board should consist of a line of Wall-nuts with a line of Chompers behind it, followed by a line of Peashooters and then, of course, your Sunflowers.
This defense can be used on any map in the game. Its big advantage is that Chompers will eat just about anything, making it very tough for zombies to make progress. In the later game this defense can be upgraded by using Tall-nuts instead of Wall-nuts. Just watch out for enemies like the Balloon Zombie or the Ladder Zombie.