Tower Defense Games

A screenshot from Defense Grid: The Awakening (2008)

Tower Defense Games or TD Games are essentially a sub-genre of strategy games. The TD Game genre is generally comprised of a human player who allocates resources to build 'Towers' in order to prevent enemies (commonly referred to as 'creeps') from passing through the human players map. In order to succeed in the TD Game genre, human players have to allocate resources in order to build defensive towers. These towers have the ability to shoot, damage, halt, and eliminate creeps from the map. Each eliminated creep results in gained resources, these resources are then stockpiled by the human player to buy more towers, or upgrade current ones. Winning is usually defined as surviving the onslaught of creeps (The human's towers eliminate all (or a sizable portion) of the creeps) and losing is defined as the creeps reaching the end of the map before your towers have a chance to take them out. TD Games are usually very easy to pick up however hard to master.

TD Games can be simple or complex, turn-based or real-time and all involve a degree strategy. They can also be single-player, cooperative, or multiplayer - varying from title to title.

Gameplay Elements

Phillipa Avery describes the 5 elements that combine to comprise an example of the TD Game in his text, "Computational Intellegence and Tower Defense Games".

Terrain

The map. Users can strategically place their towers on any part of the map. This map is usually comprised of two elements, the path and the area surrounding the path. Creeps travel along the path through the map. Towers can be placed anywhere on the terrain that is not on the path. Maps can change through different levels.

Towers

Towers are the human player's defense against the invading creeps. Normally towers can be placed anywhere on the map except directly on the path. Towers can be upgraded with resources. Towers have many different abilities, costs, and uses. They cover different ranges and have specific targets. Towers can differ in cost, range, power, firing rate, effect on creeps and more.

Creeps

Creeps are defined as the enemy. Creeps move along the path at varying speeds trying to reach the end of the path. Creeps usually have different attributes such as size, speed and defense. Usually in TD Games, the creeps that follow the path in the beginning levels are relatively low strength and low speed. As your progress through the game, these creeps increase in armor, speed and size and typically take multiple upgraded towers to kill. Some creeps are unaffected by certain towers, thus the human player is best served to diversify his arsenal of towers.

Reward Systems

These reward systems are created to, as Avery states, "...increase the interest and lengevity of TD games..." As the creeps pass though the map, the towers destroy them. As each creep is destroyed, typically, the human player is rewarded with some sort of resource. These resources can be used to purchase tower upgrades, new towers, and more. These resources function as a way for the users to increase the power of their tower defense in order to sufficiently withstand the next levels creeps. Acquiring new defenses is an imperative feature in the design of TD games and allow the player to confront increasingly difficult and/or numerous AI or player-controlled enemies. This enables developers to incorporate more variability int he gameplay-contingent on the axiom that players are acquiring the rewards.

Players

The majority of TD games involve a singe player strategizing the use of his tower defense against the computer-controlled creeps. However there are examples of multiplayer TD games such as Rampart as well as others that function to have humans work as a team to withhold the onslaught of creeps. This is also an example of a cooperative TD game. The popular multiplayer online battle arena (MOBA) game League of Legends is known for incorporating elements of tower defense into a multiplayer battle arena.

History

Early Beginnings

The Tower Defense Game genre is relatively young in the world of gaming. The TD Game genre finds its beginnings in 1990, with an arcade game released by Atari Games titled Rampart. The large success of Rampart caused the game to be released on a wide variety of platforms including Game Boy, Super NES, Playstation, Xbox, and more.

Increase in Popularity

The success of the Tower Defense model generated significant buzz in the gaming community and eventually influenced the creation of custom maps that adopted the TD Game Model for mainstream computer games such as Starcraft, Age of Empires II, and Warcraft III. Fans utilized the free map creation tools in Starcraft and Warcraft III to create custom TD style maps. These Tower Defense maps became so popular that the game developers themselves included TD style levels in further releases of the game as well as in expansion packs. As noted in Phillipa Avery's text, Computational Intelligence and Tower Defense Games, "One of the most popular versions of these [tower defense model-based maps], and arguably the original true TD style game was the Tower Defense maps for the Warcraft III expansion The Frozen Throne." This addition in the Warcraft III expansion signaled the TD Game genre's entrance into the mainstream and sparked a growth in the genre's awareness on a larger level.

Entrance into the Mainstream Market

The Tower Defense Game genre entered the mainstream market with the boom of flash-based web games. Until 2006, no one really bothered to develop a desktop-based model of the Tower Defense Game. In early 2007, a developer named David Scott launched Flash Element Tower Defense. This game was so successful that within two days, 500,000 users were playing it per day. The success that David Scott achieved with Flash Element TD rubbed off onto one of Scott's close friends, Paul Preece. Preece released Desktop Tower Defense in March of 2007 and within 4 months the game had been played over 15 million times and brought in over $12,000 in revenues. Desktop Tower Defense soared in popularity, garnering awards such as the 2008 Gleemax Award for Strategic Gameplay (“The Gleemie”) at the Independent Games Festival and it's success is credited with the launching of the Tower Defense Game genre into the mainstream.

The Mobile Platform

The Tower Defense Genre continues to be very successful on the mobile platform. Today, Bloons TD5 and Plants vs. Zombies are the leading mobile Tower Defense Games in the Apple App Store. The mechanics and environment of mobile gaming platforms such as iOS and Android are very receptive to TD videogames. Many tower defense titles have hit the top of the charts like Fieldrunners and Anomaly: Warzone Earth. The simple nature of these games makes them well-suited to be marketed at the low prices or 'free to play' models that are consistently found in the App Store or on Google Play.

Examples

Rampart

Developed and released by Atari Games in 1990, Rampart is regarded as the first game that fits the Tower Defense Game Genre. Rampart was an arcade console unit and featured four different game modes, single player, two-player harbor, two-player river, and a three-player mode. Rampart's arcade style console was an upright machine with a horizontal monitor. The control panel features optical trackballs and two buttons (fire and rotate) for as many as three players. Single player mode consisted of a castle defense unit and cannons. The Human player would have to construct his castle defense as well as place the cannons strategically on the map. The cannons could rotate and fire in accordance to the two buttons on the control panel. There were three different types of ships, with the number of shots it takes to sink a ship depending on the size of the ship. In Two-player harbor mode, the area of gameplay is divided into thirds, one third for water and ships, and the other two-thirds for each players castle. Players must sink ships and destroy the other castle to win. In Two-player river mode, the area of gameplay is split down the middle by a river. The players face each other and try to destroy the others castles and ships. Three player mode is identical to two player river mode, the area of play is divided into thirds by a Y-shaped river. Modern ports of Rampart can still be found today on various platforms from iOS to Playstation 3.

An example of the Rampart console.

Desktop Tower Defense

Developed by Paul Preece in 2007, Desktop Tower Defense is widely considered as the game that launched the genre of Tower Defense games into the mainstream. Desktop TD is a simple, easy to pick up, well-designed spin-off of the Real-Time Strategy genre and is speculated to be influenced by Rampart. Desktop TD is a flash-based game that found it's success through it's "short-feedback loop" and "5 levels of addiction as described by Daniel Renkel in his text, Desktop Tower Defense: Perfect Job. Renkel smartly points out that the human player's experience is significantly enhanced by this "short-feedback loop" in which the user can experience his fun and improvement almost instantaneously, feeding the thirst for instant gratification found in the youth of the world today. His description of the 5 levels of addiction is worth noting. The first level, states Renkel, is that as you fend off creeps, the user yields money (resources) that can be used to upgrade or purchase more defenses. The second level is that you can use these resources to buy more towers and make the path in which the creeps travels longer and harder, therefore you accumulate more resources to build more towers. The third level, is the "send next wave" button in the game. Using this, Renkel says, you can allow more creeps to pass through and build a stockpile of even more resources. Level 4 is the fact that instantly, when you lose you can start a new match and access more towers, enemies and upgrades. And finally, level 5 is the high score list, which you can compare and show off. These "5 levels" of addiction, detailed by Renkel, are ultimately what fueled the success of the Tower Defense Genre and illustrate why Desktop Tower Defense is such an important case study when looking at the Tower Defense game genre as a whole.

In an interview with the Wall Street Journal on June 20, 2007, Preece stated that he believes his creation and implementation of scoreboards as well as custom private group scoreboards was instrumental in the game's popularity. Preece also mentions that Desktop Tower Defense was his first attempt at game design. Also in the 2007 WSJ interview, Reece points out that the game had been played over 15 million times within 4 months of it's release and has generated $12,000 in revenues during those four months.

Desktop Tower Defense was awarded the 2008 Gleemax Award for Strategic Gameplay (“The Gleemie”) at the Independent Games Festival

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  Screenshot of Desktop Tower Defense Source: http://www.aspkin.com/desktop-tower-defense-most-addicating-game-ever/

Plants vs. Zombies

In 2009, PopCap Games released Plants vs. Zombies for Mac OSX and Microsoft Windows. Since it's release in 2009, Plants vs. Zombies has garnered a significant amount of worldwide attention including awards, topping charts, and receiving a large number of great reviews. Initially released for desktop platforms, the game's popularity resulted in it's release on 10 different platforms including Xbox, Playstation, Blackberry, Android, iOS, Nintendo, Kindle, Nook, and Windows Phone. The game has also resulted in sequels being released on other consoles as well.

The Plants vs. Zombies game fits the Tower Defense Genre very well. The plants (towers) can be positioned strategically along the paths and defend against the zombies (creeps). In this version of Tower Defense, the towers could be built on the paths, hindering the creeps from reaching the end. The creeps can, unlike games such as Bloons TD or Desktop Tower Defense, destroy your towers. If a single creep reaches the end of your path, the game is over. Plants vs. Zombies fit the 5 levels of addiction model detailed above by Renkel. Ctamas states that, Plants vs Zombies differs from the original TD genre in that the gameplay is focused more on creating a gauntlet for the creeps to be destroyed rather than a maze-like atmosphere.

Plants vs. Zombies was one of the first examples of the Tower Defense genre excelling on the mobile platform. Due to it's success on the desktop platform, Plants vs. Zombies was, in February of 2010, released for iOS. It's launch, at the time, was "the top-grossing iPhone launch ever" and grossed over $1 million in 10 days.

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  Screenshot of Plants vs. Zombies source:http://www.popcap.com/plants-vs-zombies-1

Plants vs. Zombies is highly decorated, it's awards include Nintendo DS Game of the Year (Game Industry News, 2012), TIME Magazine's 50 Best iPhone Apps of 2011 (TIME Magazine, 2011), Best Family Game (Xbox LIVE Arcade Awards, 2011), Top 25 PC Games of the 2000s (GameSpy, 2011), Editor’s Choice Gold Award (Game Informer, 2010), Editor’s Choice Award (IGN.com, 2009), Editor’s Choice Award (WorthPlaying.com, 2009), Editor’s Choice Award (PC Gamer, 2009), Maximum Kick Ass Award (Maximum PC, 2009), Best Strategy Game of the Year (USAToday.com, 2009), Inductee, Game Hall of Fame (MacWorld, 2009), and the Casual Game of the Year (Chicago Sun-Times, 2009).

Significance

The Tower Defense Game Genre is often described as a hybrid between two genres, Real-Time strategy Games and shooter games or even as a sub-genre of Real-Time strategy games. The Tower Defense Genre is significant in the realm of computer gaming because it is an interesting example of genre-mixing. Relatively new to the world of gaming, the Tower Defense Genre is a gateway game. The TD genre bridges the gap between arcade shooter and real-time strategy. It's addictive properties and ability to provide instant gratification have catapulted this genre to the top of the gaming totem pole almost instantly. The genre has captured the attention of such a large community of gamers and this is telling of its diverse nature and it's easy to start, hard to master gameplay.

Intrinsic Elements

The TD genre captures the nature of humans very well. The idea of protecting a "home base" or "fort" as posed by Ryan Clements of IGN, in his article titled, Why We Love Tower Defense: Staying safe from goblins and ghouls is very telling of the success of the Tower Defense Genre. As humans we want, it is our nature, to protect ourselves against the evil of the world. One we start playing games of this genre, our human nature kicks in and we are hooked, we will play and play and play some more to protect ourselves from these evils. Clements states, "This urge to protect and preserve fuels our need to keep playing. And it drives us to play well." The genre of Tower Defense games can attribute its success to striking a significant chord within us humans, our protective nature, we protect what is ours.

Instant Gratification

The Tower Defense genre enables the human player to instantly garner reinforcement and reward. In games such as Plants vs. Zombies, Bloons TD and Desktop Tower Defense the more you kill creeps the higher your score and the more resources you gain. This applies to a number of game genres, however, what makes the Tower Defense Genre all the more interesting is the "short feedback loop" described by Renkel. Human players receive positive reinforcement almost instantaneously. In the TD genre there is minimal waiting and levels can be beaten within seconds. The Tower Defense Genre satiates the thirst of positive reinforcement and gratification constantly. Human players earn resources, build more, earn more, and get better. This results in the constant gratification and reward.

Educational

The Tower Defense genre mixes shooters with strategy, and thus, a relatively diverse set of skills is required to master this genre. The Tower Defense Genre, as described by Mike Langlois in his article, Tower Defense & Executive Functioning, can act as a tool for improving many skill sets and executive functions. Langlois details how budget spending, pacing investments, and saving money are all tactics taught by the genre. Tower Defense games are not won with the typical shooting game skills such as reflex, hand-eye coordination and speedy fingers, states Langlois, "You need to be able to learn from your past experiences, and often switch strategies midway through the game. You need to recall which towers are best for different situations and monsters. There is a map to be managed in space and a marching army and builders to manage in time. You need to recognize both immediate feedback and notice trends." The Tower Defense genre implores a specific set of skills, it forces the human player to account for variables such as money, time, situation, space and strategy. The Tower Defense game genre can absolutely function as an educational tool, teaching the human player to take into consideration many different tactics and strategies.