The majority of gameplay in Plants vs. Zombies can be best described as tower defense. All settings feature six "lanes" along which various zombies will travel from the right side of the screen with the aim of reaching the left. The player's goal is to build up defensive lines of plants across the lanes such that the zombies are prevented from crossing the screen AND EATING YOUR BRAINS!
The Play Field
The play field is essentially a 5 by 9 (Sometimes 6 by 9) grid in which plants can be placed anywhere at any time. Grid squares are generally highlighted by alternating light and dark colours. Initially, the whole field is grass, which can be freely planted on, but later in the game, some rows are replaced by water (requiring the use of aquatic plants or by first placing a special plant which can then be planted upon) and close to the end, gameplay switches to a rooftop environment (requiring players to place pots before planting). Some plants should be kept away from zombies at the left side of the screen, while others should be placed closer to the right for maximum effectiveness.
Combat is predominantly row-based; if left to their own devices, zombies will move left along the lane in which they appeared. Similarly, most plants are only capable of attacking in the lane they were planted, although there are some exceptions.
Building Up Defenses
Like with most defensive strategy games, creating a strong barrier is a balance between gathering resources and creating defensive units. In Plants vs. Zombies, the main in-game resource is "Sun". Sun is used to plant more units and can be gathered from special sun-generating plants as well as having it randomly appear on the game field. Sun appears as a yellow multi-pointed star with a lighter yellow centre in two sizes. The smaller stars are worth 15 sun, while the larger stars are worth 25 sun. Gathering lots of sun is crucial to success, and some of the more powerful defensive plants cost in excess of 300 sun.
At the start of each level, the player is shown what types of zombies will appear during the level and then he/she chooses a combination of plants to combat them. Initially, only 6 types plants can be chosen, but this can be upgraded as as the player progresses through the game's main story. The player will usually start with 50 sun to begin with and a blank field. The first zombies will appear approximately 30 seconds later, accompanied by an audio cue:
The zombies... are coming...
Progress to completion is indicated by a progress meter on the bottom right of the screen. A green bar fills the meter as the level progresses. On most levels, at least one point along the bar is marked with a red flag with a brain on it, symbolising a "wave" event. At this point, a large number of zombies will appear. The game often uses this event to introduce a new type of zombie, and most levels finish with a wave event. Early levels have only the end wave, but later levels can have up to 5 wave events. In multiple wave levels, events are always spread evenly across the level.