Tower Defense games, also known as TD games, are a sub-genre of computer-based strategy games. What they have in common is the goal of surviving and earning points by building towers on the game terrain. The enemies, sometimes called creepers, then appear and your towers automatically attack them as they pass. The more enemies killed, the more points you earn; points can be used to buy new towers or upgrade existing ones. The player starts with a certain number of lives, and loses one each time a enemy escapes. When the player's life is exhausted, the game end.

Available TD games differ in terrain and the rules by which the enemy can cross it, and in the types of towers and weapons available to the player. Most games use a maze, or set of pathways that the enemies must follow, allowing the player to predict which tower positions will do the most damage. Some games allow the enemies to move in straight lines, referred to as line TD games.

Obviously, where you place your towers determines your success. However, towers vary in their range, the amount of damage they do, and which types of enemies they will fire upon. Therefore, it takes not only the right placement, but the right combination as well.

One of the earliest Tower Defense games, credited with inspiring many designers to create their own TD games, was Rampart. Rampart was originally released by Atari Games in 1990 as an arcade machine, but its success resulted in the game being ported to a number of platforms, including Super NES, Game Boy, Playstation 2, X Box, GameCube and, most recently, Playstation Network, to offer multiplayer action. In addition, elements of the Tower Defense games made an appearance in other games, such as Warcraft III and Age of Empires II.

The proliferation of TD games has produced many inventive variations on the original game, in rules, terrain, and back story. Currently popular versions are Flash Element TD, Bloons Tower Defense 2 and Pixelshocks Tower Defence. Some replace towers with characters such as wizards and archers, changing the range of possible placement and types attacks. Games may be set in a canyon, the sky, or other settings. Versions for children use the scenario of protecting picnickers from ants, while a dungeon version introduces zombies, wraiths, and other traditional dungeon dwellers.

Recently, a Flash-based browser version of Tower Defense, called Desktop Tower Defense, has become a runaway hit. Designed by first-time designer Paul Preece, it was named one of Webware 100's top ten entertainment web applications of 2007, and averages 20 million pages views every month. Desktop TD has three difficulty levels, and several challenge modes that present a difficult set of circumstances that create a problem to solve. Challenges include one called The 100", where the player must defeat 100 levels of increasingly strong and crafty enemies, and "3K Fixed", where the player must work with a fixed budget of 3000 dollars.

For those interested in trying Tower Defense games, there are over 100 free versions, both web-based and for download. Onslaught and Desktop Tower Defense are free web-based versions that are frequently recommended by Tower Defense aficionados. If you're looking for a downloadable game that you can play off-line, Wicked Defense, Master of Defense, and Zombie Defense will all provide a good introduction to the genre.