Tower Defense
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Tower Defense (TD) is a game type, common to certain subsects of the real-time strategy genre, whereby players create units called "towers" in order to stop non-player-controlled enemies (creeps) from reaching a certain goal point.
Tower Defense game types are extremely popular "custom" maps in Age of Empires II, StarCraft, Warcraft III, and Command & Conquer: Generals apart from the regular game. "Photon Defense" is the archetypal Tower Defense map for StarCraft; Warcraft III has "Wintermaul" (which was followed by many other "mauls" and also a competitive version, noted with the addition of war in its name), "Line Tower Wars", and "Insane Tower Defense." The variations, of course, are endless.
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Concept
A typical "Tower Defense" game would consist of a flat playing field, with clearly marked, distinct pathways which the non-player controlled enemies follow to reach the goal. Enemy units are created at set intervals at a set starting point; they then travel along the marked pathways towards the goal point. Typically, enemy units are divided up into "waves." Each wave consists of the same type of unit, though unit combination waves may also exist. Players build towers alongside or on (but not blocking) the pathways, attempting to destroy the enemy units that pass by. Blocking the pathways is considered an act of cheating (no waves passing means the player will not lose); therefore players who block are generally penalized, for example by having their towers destroyed. Each subsequent wave has tougher or faster units, so players must continue to build and upgrade to new units and towers or risk being overrun. There are a finite number of waves, usually around 20-50, depending on the specific scenario, and if all the waves are destroyed then the players win. If enough enemy units make it to the goal point (this number also varies per scenario), then the players lose.
Gameplay
Certain strategic and tactical properties remain inherent across all Tower Defense game types. Players start with a sole worker, who is then used to build different types of towers. They have limited resources, so they have to pick and choose what type of towers to build, and where to build them. Corners, where the pathway enemy units take turns, are often the most lucrative areas to erect tower fortifications. If players are allowed to build on the pathway itself (to construct their own pathways for the waves, in other words), a practice called "mazing" is adopted, where they build towers into mazes, in order to maximize the time enemy units spend in the maze. They also will use "looping" (making a maze where a creep must go to a waypoint and go back through the maze to get to the next waypoint). One must destroy enemy units, in order to collect their resource bounties. Upgrading to new and better towers, or new and better units, is necessary to defeat the waves as they grow stronger and your towers get relatively weaker.
General categories
This entry is only a general outline of Tower Defense, which can be exceedingly complicated in all its intricacies. Making generalizations about Tower Defense is a practice also fraught with danger, due to the extreme mutlitude of different Tower Defense scenarios currently in existence. However, in general, tower defense can be divided into two categories. Generally, a Tower Defense refers to a map where players cannot build on the paths the waves travel along, while a 'Maul' allows this privilege, and the map shapes vary accordingly.
Cooperative
In this mode (Players vs Computer), players build towers to destroy computer-controlled waves. It is cooperative in the sense that the players do not fight each other; or rather, they work together to win the game in some way or another. One must work with other players to take down the waves, for no one player can win any cooperative Tower Defense by himself.
There are a few variations to cooperative Tower Defense. Commonly it is simply protecting a goal from being reached by computer units, where the players have a "shared life" (in other words, all players have only a specified number of "lives" - total enemy units that can reach the players' goal - before all players lose the game). The map can be designed so that there are separate waves for each player, for instance one in each corner of the map. An alternative map design would be all players stopping the creep waves by building in a pattern (like in the Wintermaul series - waves come from the top and will pass certain pathways; all the players must prevent them from reaching the bottom by building towers in the middle according to their respective pathways). One running joke in Warcraft III Tower Defenses is to have Timmy, a character from the campaign, be the super-powerful final wave.
An interesting variation on Tower Defense involves a scenario where enemy units focus on a single player-controlled unit, trying to destroy it. This unit is typically quite powerful. Other players build towers to protect the unit. If it dies, the players lose. Enemy units also spawn in waves. Other variations include enemy waves being able to attack the player towers (in regular Tower Defense they do not).
Competitive
Competitive tower defense (Player vs Player and/or Computer) has all the basic concepts of a Tower Defense game; however several elements are added to give it a competitive edge.
Some variants are like Cooperative tower defense, however, each player has their own counter of "lives" instead of "shared life". An individual player is given a number of "lives", which represent the total number of enemy units that can reach their goal point before they lose. A loss by an individual player does not affect the score of others. The winner is determined by the "last man standing".
The Line Tower Wars series is a popular competitive Tower Defense map, which involves players spawning their own waves in order to destroy other players (instead of playing against a non-player controlled entity, players in this game type play against each other). Players in Line Tower Wars spawn their own waves in order to generate bounty, as well as to bypass their enemies Towers to reach a goal point. (The Line Tower Wars have spawned a new type of custom map, Hero Line Wars).
Another variant involves players fighting against both non-player controlled enemy waves and each other at the same time.
A common complaint against all TD style maps is that air units do not follow the mazes players can create with their towers, and will frequently go right over a player's strongest unit. Often times a person hosting an instance of a TD will specify No Air, and it is generally accepted that as long as no one summons air first, it will never be summoned. Some people will edit existing TD's only to cripple or remove the air units, allowing for a longer game play.
The Wintermaul Wars series is a prime example of this. Many newer versions for the The Frozen Throne(TFT) version of this map allow players to vote on whether or not air should be allowed. Though playing to defend against air and ground is part of the game, some players have strategies that are not equipped to deal with this.
In addition, in some TDs players may vote on options such as life steal (for every monster that gets through the opponent's defense, your team gains a life and the other team loses a life).
The Crossing
WarCraft III: Reign of Chaos was particularly popular for Tower Defence maps, to the point where the secret level 'The Crossing' in the games' expansion pack, WarCraft III: The Frozen Throne was an in-game Tower Defence level, which vaguely fit into the campaign's story.