Tower defense

**Tower defense** is a sub-genre of real time strategy games. The player is tasked with defending a goal from enemies, which are released from a point usually opposite that goal on the screen or the path (if a fixed path exists). To achieve this, players place the titular "towers" that use attacks, abilities, and sometimes even their own durability to prevent (or try to prevent) the enemies, from reaching their goal.

All *Plants vs. Zombies* and *Plants vs. Zombies 2*) are examples of tower defense.

## Tower types

### Direct damage

Direct damage towers are the bread and butter of all tower defense games. They basically have one purpose: to shoot at a target in range until it dies.
Their main priorities are: damage per second (DPS), in some cases damage per shot (if for example some units have a flat reduction to damage), range (so they can shoot for a longer time), and projectile speed (to reduce overkill if applicable).
Examples of 'pure' direct damage units in Plants vs. Zombies include: Peashooter, Repeater, and Puff-shroom.
### Area-of-effect damage

Area-of-effect (AoE) towers generally cost more or do less single-target damage, but as the name implies, they target all enemies within a radius of the target, or all enemies within range of the tower itself. This makes them essential to dealing with large groups of enemies that direct damage towers do not have the DPS to handle.
Their main priorities are: feasible damage per second (basically a function of how large the area of effect is and how much single-target damage per second) and range.
Examples of AoE units include: Melon-pult, Fume-shroom, and Gloom-shroom.
### Defensive tower

Defensive towers usually have no offensive abilities of their own, but exist to block the enemy's progress and/or protect more vulnerable towers.
Their main priorities are durability and health, and cost-effectiveness.
Examples of defensive towers include: Wall-nut (pure), Tall-nut (could also debuff), and Garlic (also has a diverting ability).
### Buffing tower

These towers increase the effectiveness of other towers in the vicinity, either by increasing their durability or power, or by decreasing their vulnerabilities.
Their main priorities are effective area, strength of buff, and rate of buff.
Examples include Torchwood and Pumpkin.
### Debuffing towers

These towers decrease the enemy's strength and or advantages.
Their main priorities are strength of debuff, duration of debuff, and rate of applying debuffs (and area of effect, if applicable).
Examples include:  Tall-nut (removes vaulting objects), Sap-fling, and Magnet-shroom.
### Environment modifiers

These are towers that modify the environment in a way; usually to benefit it for other towers.
Some examples of environment modification include removing obstacles, allowing more places to create towers and to stop ways of enemy approach.
Examples include: Grave Buster, Lily Pad and Plantern.
### Casters

These generally perform their duty once and disappear, or in rarer cases, have some effect on the environment.
Usually they have a large area of effect or are very powerful in a small area.
Examples of casters include: Blover, Cherry Bomb, Squash, Jalapeno, Potato Mine, Ice-shroom, Grave Buster, Coffee Bean, and Hypno-shroom.
### Energy/income

These units create energy, wealth, or points that the player needs in order to purchase items/towers, progress in a level, or progress in the game itself.
They are generally useless aside from this purpose, so if the game allows them to be attacked (which does happen in this game), protecting them is of utmost importance.
Examples of energy units include: Sunflower, Sun-shroom, Twin Sunflower, and Marigold.
### Upgrades

These towers are never "pure" in any aspect, but are placed on a specific already existent tower to improve their strength, fire rate, add a special ability, or completely change the uses for the tower.
Examples of upgrades include: Gatling Pea, Twin Sunflower, and Spikerock.
## Pure vs. mixed units

Typically in a tower defense game there are units that offer mixed roles in the game. Usually the tradeoff is that the more skills a unit has, the less proficient it is in each of them, and/or the more expensive it is to purchase or to improve itself.
Here are a few mixed units in *Plants vs. Zombies*.

* Ice-shroom: caster and debuffer
* Snow Pea: enemy debuffer and direct damage
* Tall-nut: defensive tower and enemy debuffer
* Kernel-pult: enemy debuffer and direct damage
* Winter Melon: enemy debuffer, area-of-effect damage, and upgrade
* Doom-shroom: caster and environment modifier
* Blover: caster and environment modifier
* Spikerock: area-of-effect damage, defensive tower, and upgrade
* Twin Sunflower: energy/income and upgrade