A game where you defend a building from monsters using other buildings, sometimes with a unit or two to back you up. Similar to Multi-Mook Melee, Tower Defense games have you facing wave upon wave of "creeps"note ("Creeps" being the term used for random monsters in Warcraft 3; as a lot of Tower Defense maps were made for WC3, the name stuck.) until a given number reach their goal, or you survive the final wave.
Creeps will move along the path that is either:
Fixed: Creeps move along the same paths.
Dynamic: The placement of towers will determine the paths which creeps move along. Dynamic paths usually create a strategy called "mazing", allowing the player to make long, winding paths that force the enemy to take the longest possible route to their destination, all the while being blasted by the towers herding them.
A strategy that requires a balance of producing more towers and upgrading existing ones. More important in dynamic path type ones.
Balance between different towers. Ideally, none of the tower types should be completely useless.
Commonly, a "survival" mode in which each level retains the towers you built in the previous level and/or the enemies get tougher as the game progresses instead of simply increasing in number.
As of lately, a trend in this genre has started to develop where the games will feature the ability for the player to be the attacking force pitted against the towers. Some games in the genre get to the point of devoting themselves entirely to this, eschewing the defense element altogether.
Age Of Empires II: Age Of Kings (and the expansion pack Conquerors) had a "Wonder Race" game type, in which you are required to build a wonder and defend it for 200 years before your opponent(s).
Civilization IV came with a "Civ Defense" mod, in which one starts with a certain number of cities, spends money (rather than the usual "hammers") to add to their fortifications and defenses, and then horde after horde of barbarians come at you.
Final Fantasy VI has has an event similar to this halfway through the first part of the game; there's a difference in that you battle the mooks using the standard battle system, but otherwise the execution is the same.
The Trope Maker was the various user-made defense maps in Starcraft - Sunken D, Turret D, Stacked Photon D, etc. The Starcraft editor did not allow you to create new units or buildings, so they usually made much more use of mobile units than most current Tower Defenses. See, for example, this video of Weed D, which is recognizably a fixed-path Tower Defense game except that the "towers" being "built" are Mutalisks.
The Trope Codifier was Tower Defense maps for Warcraft III. The editor was much more sophisticated than Starcraft's, allowing for greater variety in attack buildings. Multiple subgenres appeared and proliferated, such as the Wintermaul clones.
The expansion pack, Frozen Throne, contains a Tower Defense map as a bonus level.
A variation of this appears in Legion's Loyalty Mission in Mass Effect 2, where Mooks must march towards you in a fixed path while you have Legion hack a few turrets around the area to shoot at them. Alternatively, you can just cap them yourself.
Mann Versus Machine mode in Team Fortress 2 is something of a First-Person Shooter version of a Tower Defense minigame. There is/was also a fan-made Tower Defense minigame mod in development.
"Red's Mighty Feathers" from Angry Birds adapts the Angry Birds formula for the Tower Defense genre. The pigs are advancing upon the egg using their vehicles, and the Red Bird must protect the egg by popping the pigs with his new homing power.
Examples of Tower Defense games that include the ability to play as the attackers:
Villainous - Also a Tower Offense game where you play as the villain on his quest to take over the lands.
Armored Core V - A Mecha Game where most of the online component consists of you assaulting other team's territories akin to a glorified 3D-Tower Defense game. As with everything else however, being intercepted by that territory's owner and prepare for a Team Deathmatch style game instead, with all the turrets present. Like real Turret Defense games, turrets run the gamut from "squishy little targets" to "hard as a barnacle to remove", and combinations of turrets often give even the most experienced players trouble.
A game literally named Anti-Tower Defense. You select versions of certain robot creeps to casually walk (argh) through a progressively harder tower network.
Plants vs. Zombies on the Xbox 360 has the 2-player versus mode. The zombies' side has you attacking the plants and attempting to get into the house, while defending your targets from the plants' attacks. All the versions of the game have a simpler version of this in the "I, Zombie" minigame, where you have to pick zombies to attack a pre-selected set of plants.
Stronghold, a series of castle-building games which are mostly defensive except for the rare mission where you play an invading army trying to get into an opponent's castle. These also involve setting up a complex supply chain, managing taxes and morale, but for the most part, the aim is to build a big wall around your keep, and stock it with as many archers and crossbowmen as possible to pick off the approaching invaders.
Dungeon Defenders has an event match called 'assault' where, "in an unexpected twist, YOU have to attack THEIR crystal!"
Doggnation was originally designed as a Tower Defense game, but this eventually changed into more of a puzzle game where you have to help the "Doggs" carry specific blocks to upgrade their castle.
Dead Space Ignition has a Hacking Mini Game that is a tower Offense game. You send out an unlimited number of computer viruses to break down the computer firewalls and get to the computer program before time runs out.