There is a great deal of strategy which you'll have to employ as you progress to harder levels since you can only bring in 7 different plant types for a particular level. This gets intricately more complicated though, because there is no way of knowing which path you should cover first or the variety of zombie types which will rush down a certain path. It seems like a simple concept: select from a number of plants, each with a different way to take down the zombies, place them along a certain path, and then wait until you can purchase more plants by collecting the sunray resources. Figure out which plants work best against the 26 different zombie types, such as pole vaulting zombies which jump over bomb plants or mining helmet wearing zombies which take more damage.