Tower Defense (TD) games appeared in Flash (browser-based) form around 2006. Precursors to the style include Rampart and Defense of the Ancients, neither of which I have played. Flash Element TD was the first web-based entry to make a splash (it may have been the first Flash TD game, in fact).

Tower Defense games fall into the broader genre of strategy games, a category that can also be said to include the following:

As Wikipedia’s article about strategy games points out, most games include strategic elements. Genres are rarely “pure,” and even many of the games above incorporate luck, skill and other non-strategic aspects. What separates strategy games, in my mind, is that they rely primarily on clearly defined set of rules, which are ideally apparent to the player or players from the start. Gameplay consists of making the most efficient choices within these rules.

The recipe

Tower Defense games usually include the following elements:

  • a top-down view of a limited field of play
  • waves of enemies that enter the field of play at a specific location, and move towards another location, at which point they exit the field of play
  • a limited number of player “lives,” which are lost when enemies are able to exit the field of play (the game ends when all lives are lost on a specific field of play)
  • the ability for the player to construct “towers” that damage enemies in various ways, thus preventing them from reaching the exit
  • the ability for the player to improve his defenses by upgrading existing towers or researching new types of towers
  • limited resources, which are replenished by destroying enemies, and which are “spent” on tower construction, upgrades or research
  • various enemy types with specific traits, against which different types of towers are effective to a greater or lesser degree

Common variations on the theme include the following:

  • fields of play in which the path that enemies will take is determined by the location in which towers are placed (as opposed to a predefined enemy path bordered by areas in which towers may be placed)
  • branching upgrade paths within a single tower type
  • the presence of rpg-like leveling mechanics, wherein towers cannot be upgraded until they have done enough damage or destroyed enough enemies
  • multiple fields of play, which may or may not be accessible from a “world map” screen
  • earned advantages that carry over from surviving previous encounters
  • the ability to speed up or overlap enemy waves for a greater challenge and to earn additional points or resources

When broken down like this, there appears to be very little room for variation within the genre, and I’ve certainly heard the sentiment that “if you’ve played one tower defense game, you’ve played them all.” I think this is exactly as true of TD games as it is of first-person shooters, point-and-click adventure games, rogue-like RPGs, room escape games, or any other genre defined with reasonable precision.

Where’s the fun?

This is a subjective question, but it needs to be addressed. I believe that the fun in these games comes from seeing how long you can survive—how effective your strategy has been—and from the satisfaction of improving your towers so that they more effectively mow down the enemy.

Finding an effective tower setup is an iterative process. The player does not only need to determine the optimal final setup, but the optimal order in which to build and develop towers based on the order in which enemy types will appear. Towers may fire slowly or quickly, and they may only be effective against ground or air enemies. Some towers alter enemy speed, or do splash damage to all nearby enemies.

Many TD games present “boss” waves at regular intervals, during which a single powerful enemy replaces a horde of weaker ones. This, too, changes the calculus of the game. As a bigger man than I once pointed out, “you use different moves when you’re fighting half a dozen people than when you only have to be worried about one.” Bosses are in some ways less dangerous than waves of regular enemies, since they only dock the player one life if they are not defeated. However, failing to earn the extra resources that destroying them would have yielded can cripple the player down the line.

As with any other game, the player’s enjoyment of a TD game will depend in large part on how she feels about the setting or trappings of the game. TD games do not generally include immersive narratives, so in practice this often comes down to a question of theme. Fantasy and science fiction themes abound, but so do cartoonish themes like those of Bloons Tower Defense and Mushroom Revolution.

One final note on the question of where the game lies in this genre: two tower defense games with identical rules may play very differently, because so much depends on the play field layout and the order and strength of enemies. The more tightly tuned a TD game is, the fewer options exist for a successful development path and final tower layout, and the more that game will play like puzzle game. Because the player’s iteration is based on trial and error, this approach can feel punishing. Although I haven’t got the data for a study, I’d wager that the most successful TD games strike a balance between challenging players and demanding one of a very few “right answers.”

Potential educational value


Super Energy Apocalypse Recycled!

From an educational standpoint, I believe that strategy games can be a wonderful way to introduce and improve systems thinking.

The fact that the rules tend to be clearly defined does not mean that they must be simple, or that multiple systems cannot exist within the game, interacting with one another in complex ways.

TD games are probably not a fabulous educational tool, however. Concepts including resource management and efficiency are touched upon through gameplay, but the variables over which the player exercises control are so limited, and the trappings and mechanics of the games are so far removed from the world around us, that in my opinion any skills gained in these areas are not very transferrable. Perhaps players do learn that it is important in general to manage limited resources efficiently.

Despite this, I should note that at least one attempt at an educational TD game has already been made! Super Energy Apocalypse: RECYCLED! addresses energy issues and, tangentially, environmentalism. The game adds a narrative and a negative resource (pollution) to the TD format, but the central mechanics are the same. Thus far I’ve only played the game a bit, and I’m not hugely impressed, but I’ll return with a more substantive look once I’ve finished it.