Tower defense (TD) is a subgenre of strategy video game where the goal
is to defend a player's territories or possessions by obstructing the
enemy attackers, 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,
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.
Ryan Clements of
IGN attributes the popularity of such games to
psychology and human vulnerability. Tower defense, according to
Clements, "plays off of our need for security, ownership, and a desire
to protect the people closest to us" arising from a need to create
intrinsic value through "ownership over things", "personal space" and
to "repel our fears and insecurities".
1.2 Modern genre emerges
1.3 2007-2008 boom
1.4 A new breed of 3D games
3 USPTO trademark
Missile Command (1980) (running on the Atari 5200) was the first
popular game to include the key elements of tower defense strategy
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 which could be used to
strategically, to obstruct enemy attacks on the player and assist the
player to defend their territory, though not specifically to protect
the territory. The 1980 game
Missile Command changed that by
introducing a strategy element. In the game, players could obstruct
incoming missiles, and there were multiple attack paths in each attack
Missile Command was also the first of its kind to make use of
a pointing device, a trackball, enabling players to use a crosshair.
The innovation was ahead of its time and anticipated the genre's later
boom, which was paved by the wide adoption of the computer mouse.
Additionally, in Missile Command, the sole target of the attackers is
the base, not a specific player character. For these reasons, some
regard it as the first true game in the genre.
While later arcade games like Defender (1981) and
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
PC gaming began. Players were now able to choose from
different methods of obstructing attackers' progress.
Green House, a popular 1982 handheld game by Nintendo
Nintendo's popular 1980s Game & Watch hand held 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, had 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 multi-task. 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 the technical limitations, simplistic
gameplay, and the rise of personal computers and handhelds the Game
Boy saw a general decline in fixed cell games and likewise the genre.
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.
Modern genre emerges
Rampart, released in 1990 is generally considered to have established
the prototypical tower defense. Rampart introduced player placed
defenses that automatically attack incoming enemies. In addition, it
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
Final Fantasy VI (1994) and the Fort Condor minigame
Final Fantasy VII (1997), which was also one of the first to
feature 3D graphics. As Real Time Strategy games gained popularity in
PC gaming, many introduced tower defense modes in their gameplay,
particularly in multiplayer modes. The 2006 maps Element Tower Defense
(Element TD) and Gem Tower Defense released in February for the
popular RTS title Warcraft III: Reign of Chaos almost single-handedly
rekindled the genre. These titles would also bring role-playing
elements to the genre for the first time.
Between 2007 and 2008, the genre became a phenomenon, due in part to
the popularity of the tower defense mode in real time strategy games,
but mainly due to the rise of
Adobe Flash independent developers as
well as the emergence of major smartphone app stores from Apple and
Google. The first stand-alone browser games emerged in 2007. Among
them were the extremely popular titles Flash Element Tower Defense
released in January,
Desktop Tower Defense
Desktop Tower Defense released in
March and Antbuster released in May. Desktop Tower
Defense earned an
Independent Games Festival
Independent Games Festival award, and its
success led to a version created for the mobile phone by a different
developer. Another significant Flash title released in 2008 was
Handheld game console
Handheld game console were not ignored in the boom and
Lock's Quest and
Ninjatown released in September and
With the arrival of Apple's App Store tower defense developers adapted
quickly to the touchscreen interface and the titles were among the
most downloaded, many of them ported directly from Flash. Among the
more notable include
Bloons TD 4
Bloons TD 4 (2009) which sold more than a million
copies on iOS.
The genre's success also led to new releases on PC and video game
consoles. Popular 2008 titles included
PixelJunk Monsters released in
January, Defense Grid: The Awakening and
Savage Moon in
Plants vs. Zombies
Plants vs. Zombies released in May 2009 was another
highly popular tower defense which became a successful series on
mobile devices. 
A new breed of 3D games
By the end of the boom, most tower defense games were still stuck in
the side scrolling, isometric, or top-down perspective graphical
medium. Iron Grip: Warlord, released in November, 2008 unsuccessfully
pioneered the first person perspective shooter with the genre. The
awkward combination of experimental tower defense mechanics with 3D
graphics was not well received, but later titles refined its execution
paving the way for a popular new breed of games. Dungeon Defenders,
released in October 2010, was one of the first tower defense games to
bring the genre to the third person perspective. It sold over 250,000
copies in first two weeks of release and over 600,000 copies by
the end of 2011. The 2011 title Sanctum, and its 2013 sequel
popularized the first person shooter hybrid that was pioneered by
these earlier games.
Anomaly: Warzone Earth released in 2011 introduced a variation of
gameplay which has been described as "reverse tower defense",
"tower attack", and "tower offense". In the game, the player
must attack the enemy bases protected by numerous defenses. Sequels
and other games have since experimented further with both styles of
With the advent of social networking service applications, such as the
Facebook Platform, tower defense has become a popular genre with
titles such as Bloons TD and
Plants vs. Zombies
Plants vs. Zombies Adventures making the
transition to turn-based play.
A screenshot of
Defenders of Ardania
Defenders of Ardania showing the genre's
characteristic towers, as well as units and a castle that serves as an
The basic gameplay elements of tower defense are:
territories or possessions (or collectively the "base") that must be
defended by the player
the base must survive waves of multiple incoming "enemy" attacks
placement of "Tower" elements, or obstructions along the path of
What differentiates tower defense from other base defending games
(such as Space Invaders) is the player's ability to strategically
place or construct obstructions in the path of attacking enemies.
In Tower defense, unlike the base, the player's main character is
usually, but not always, invincible, as the primary object is the
survival of the base rather than the player.
Some features of modern tower defense:
Player placed obstructions that can damage or kill enemy attackers
before destroying the base
Ability to repair obstructions
Ability to upgrade obstructions
Some sort of currency with which to purchase upgrades and repairs
(this can be time, in game currency or experience points, such as
being earned by the defeat of an attacking unit
Enemies capable of traversing multiple paths
Each wave usually has a set number and types of enemies
Many modern tower defense games evolved from real-time to turn based
gameplay in which there is a cycle including distinct phases such as
build, defend and repair. Many games, such as Flash Element Tower
Defense feature enemies that run through a "maze", which allows the
player to strategically place towers for optimal effectiveness.
However, some versions of the genre force the user to create the maze
out of their own towers, such as Desktop Tower Defense. Some
versions are a hybrid of these two types, with preset paths that can
be modified to some extent by tower placement, or towers that can be
modified by path placement. Often an essential strategy is "mazing",
which is the tactic of creating a long, winding path of towers to
lengthen the distance the enemies must traverse to get past the
defense. Sometimes "juggling" is possible by alternating between
barricading an exit on one side and then the other side to cause the
enemies to path back and forth until they are defeated. Some games
also allow players to modify the attack strategy used by towers to be
able to defend for an even more reasonable price.
The degree of the player's control (or lack thereof) in such games
also varies from games where the player controls a unit within the
game world, to games where the player has no direct control over units
It is a common theme in tower defense games to have air units which do
not pass through the layout of the maze, but rather fly over the
towers directly to the end destination.
Some tower defense games or custom maps also require the player to
send out enemies to their opponents' game boards respectively their
controlled areas at a common game board. Such games are also known as
tower wars games.
On June 3, 2008,
COM2US Corporation was awarded the trademark for the
term "Tower Defense", filed on June 13, 2007 – serial number
3442002. The corporation is reported to have started enforcing the
trademark: in early 2010, developers of games on Apple's App Store
reported receiving messages requiring name changes for their games,
citing trademark violation. Adding the phrase "Tower Defense"
(in capital letters) to the description of an app submission to
iTunesConnect and the app store automatically triggers a warning that
the submission is likely to be rejected for use of the term; however,
writing the phrase in lower case is still acceptable as "tower
defense" is a valid description of a game style.
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