Tower defense (TD) is a subgenre of strategy games where the goal is to defend a player's territories or possessions by obstructing the enemy attackers or by stopping enemies from reaching the exits, usually achieved by placing defensive structures on or along their path of attack. This typically means building a variety of different structures that serve to automatically block, impede, attack or destroy enemies. Tower defense is seen as a subgenre of real-time strategy video games, due to its real-time origins, even though many modern tower defense games include aspects of turn-based strategy. Strategic choice and positioning of defensive elements is an essential strategy of the genre.

The tower defense genre can trace its lineage back to the golden age of arcade video games in the 1980s. The object of the arcade game Space Invaders, released in 1978, was to defend the player's territory (represented by the bottom of the screen) against waves of incoming enemies. The game featured shields that could be used to strategically obstruct enemy attacks on the player and assist the player in defending their territory, though not to expressly protect the territory. The 1980 game Missile Command changed that by giving shields a more strategic role. In the game, players could obstruct incoming missiles, and there were multiple attack paths in each attack wave. Additionally, in Missile Command, the targets of the attackers are the bases and cities, not a specific player character.

While later arcade games like Defender (1981) and Choplifter (1982) lacked the strategy element of Missile Command, they began a trend of games that shifted the primary objective to defending non-player items. In these games, defending non-players from waves of attackers is key to progressing. Parker Brothers' 1982 title Star Wars: The Empire Strikes Back for the Atari 2600 was one of the first tie-ins to popularize the base defense style. The concept of waves of enemies attacking the base in single file (in this case AT-ATs) proved a formula that was subsequently copied by many games as the shift from arcade to PC gaming began. Players were now able to choose from different methods of obstructing attackers' progress. Sorcerer's Apprentice for the Atari 2600 featured Mickey Mouse and was first published in 1983.

Nintendo's popular 1980s Game & Watch handheld games featured many popular precursors. With their fixed sprite cells with binary states, games with waves of attackers following fixed paths were able to make use of the technical limitations of the platform yet proved simple and enjoyable to casual gamers. Vermin (1980), one of the first, tasked players with defending the garden (a theme followed by many later games) from relentless horde of moles. The following years saw a flood of similar titles, including Manhole (1981), Parachute (1981), and Popeye (1981). 1982 saw multiple titles with the primary object of protecting buildings from burning: Fire Attack, Oil Panic and Mickey & Donald. The later titles utilized multiple articulating screens to increase the difficulty for players. With two screens these games introduced basic resource management (e.g. oil and water), forcing players to multitask. Green House (1982) was another popular two screen game in which players use clouds of pesticide spray to protect flowers from waves of attacking insects. Despite the early rush of archetypal titles, ultimately there was a general decline in fixed-cell games, due to their technical limitations, simplistic gameplay, and the rise of personal computers and handhelds the Game Boy; correspondingly, this genre also declined. A rare exception was Safebuster (1988 multi-screen) in which the player protects a safe from a thief trying to blow it up.

By the mid-1980s, the strategy elements began to further evolve. Early PC gaming examples include the 1984 Commodore 64 titles Gandalf the Sorcerer, a shooter with tower defense elements, and Imagine Software's 1984 release Pedro. Pedro, a garden defense game, introduced new gameplay elements, including different enemy types as well as the ability to place fixed obstructions, and to build and repair the player's territory.

Rampart, released in 1990, is generally considered to have established the prototypical tower defense. Rampart introduced player-placed defenses and has distinct phases of build, defend and repair. These are now staple gameplay elements of many games in the genre. It was also one of the first multiplayer video games of its kind.

While Rampart was popular, similar games were rarely seen until the widespread adoption of the computer mouse on the PC. The DOS title Ambush at Sorinor (1993) was a rare exception from this era. Tower defense gameplay also made an appearance on consoles with several minigames in the Final Fantasy series, including a tower-defense minigame in Final Fantasy VI (1994) and the Fort Condor minigame in Final Fantasy VII (1997), which was also one of the first to feature 3D graphics. Attack of the Mutant Penguins for the Atari Jaguar and MS-DOS was released in 1995. Dungeon Keeper (1997) had players defend the Dungeon Heart, a gigantic gem at the centre of your dungeon, which, if destroyed, would cause the player to lose the game. Fortress was released for the Game Boy Advance in 2001.

As real-time strategy games gained popularity in PC gaming, the easy proliferation of user-created scenarios on Battle.net expanded the genre rapidly. Custom maps for StarCraft: Brood War such as Turret Defense (May 2000) and Sunken Defense (November 2001) were early examples of the genre that retained popularity for decades. Both were released early in the game's lifecycle and had various versions evolve in parallel over time due to the open-source nature of maps.