Tuesday, January 22, 2013

An Outline of a Successful Tower Defense Game – Part 1


Hi All,

Tower Defense (TD) is a popular genre of games for both the mobile and the PC world. The underlying idea is similar in almost all the TD games although some variations exist. The player is faced with a map which has an area (one ore more) from which critters (monsters) emerge and an area to which the critters are trying to get. In some variations of the game the critters try to steal some resource from the target area and bring it back to the area from which they emerged. The critters usually come in waves of ever increasing strength. The player must build defensive towers along the map to shoot or otherwise harm the critters so that they not achieve their goal. In most cases the player has one or more limited resource at his disposal but killing a critter grants the player a small amount of this resource. Sounds simple? It is simple! This is exactly why TD games are so popular, fun, and addictive.

During the past five years I have been playing numerous version of this simple concept. Over the time I came to realize that some of the TD games are much less fun than the others and I asked myself “why?”. In this blog post I try to all the properties that make a TD game fun and loved by players. The analysis is based mainly but not only on the most successful TD games on the internet: Kingdom Rush, Cursed Treasure: Don’t touch my gems!, The Protector series, The GemCraft series, Balloon TD series, Flash Element TD, Desktop TD, and many others.

If you want the key takeaway from this post then perhaps the two most important tips are:

  1. Don’t make random maps. Each map must be carefully planned for the upgrades available for the player in that level and the style of the game.
  2. Don’t confuse the player, too many tower types, too many upgrades, too complex construction scheme, too many critters, etc… In a successful TD game simplicity and clarity is the key.

Map Style

There are two main map styles:

  • Open map – The map is an open field of some discrete grid. The player can place towers on the field itself blocking the critters’ path effectively creating a maze of death through which the critters must pass. In this map type the critters keep trying to find the shortest route to the target zone while the player is not allowed to close the maze in such a way that no such route will exist. This map type allows a special technique called critter herding or juggling. With this technique you create a maze with two possible exits, when the critters rush toward the first exit you block it, when they rush toward the second exit you unblock the first and block the second and so on.
  • A path – The map contains a pre-defined path from the source of the critters to the target area. The path may have splits but the critters will always follow the path. Usually the path is somewhat wide and the critters can populate it in a non discrete fashion. In this map type the towers are built outside the path and may not block the path in any way.

DesktopTdCritters follow a simple maze in Desktop TD.

There are TD games (e.g. Canyon Defense) that have a mixed model but even in these games the path model is the main one.

My observation: It seems that the map style is not a huge factor in making a TD successful or not but it is clear that path based TD games are the vast majority of all the successful TD games. This observation supports the key takeaway (1) that a map must be carefully planned by the game developer and not left random to the player.

For both map styles there is a tower placement scheme. In some TD games it is possible to build towers in specific predefined areas/squares/places while other areas are blocked for constructions by some objects or scenery. Cursed Treasure, for example, allows building specific towers in specific land types thus controlling the tower placement but still allows for some freedom on where you may put the towers.

My observation: The best TD games limit the tower placement areas in some sense or the other. If the maps are planned carefully then it is possible to limit tower placement to specific locations, otherwise it is best to have some limitation which is reasonable for the map.

Towers and Upgrades

The main defense force of every TD game is the towers. Towers come in all shapes and sizes, from arrow shooting archers to fast firing machine guns, from monkeys throwing darts to giant rocket launchers, you name it. All TD games support upgrading the towers in one way or the other, some allow multiple upgrade paths which allow your tower to become more effective in certain locations on the map.

My observation: Don’t have more than 5 tower types. Most successful TD games have between 3 and 5 tower types. More tower types confuse the players and usually don’t contribute anything to gameplay.

BalloonTD5Balloon TD 5 has 17 tower types

Tower upgrades make towers more effective in their ability to dispatch critters. Upgrades usually affect the basic characteristics of the tower (e.g. range, firing speed, and damage) but may also allow the tower to create some special effect on the critters it hits (e.g. poison damage, slow, freeze, etc...). In most cases the upgrade is done directly on the tower by spending some resource but in some cases it can be done by attaching an external element (e.g. fire gem stone) or by another supporting tower. Some TD games allow the upgrade of a tower based on the tower experience points (XP) and not simply by using a player resource. The tower gains experience by killing critters and this experience can be spent on upgrading the tower. Note that this upgrades scheme requires careful map planning since it sometimes creates a large gap between tower capabilities over the entire map.

My observation: Don’t have multiple upgrade paths for the tower, at least not for the first couple of upgrades. At the end of the upgrade line you may allow two possible paths. For example a fast firing tower may upgrade fire speed and damage for 3 times and then chose to become a tower that specializes in long range or in an even faster firing rate.

My observation: Prefer upgrading the tower itself and not by adding some supporting tower or other type of attachments.

My observation: If you chose to make tower upgrades available based on the tower XP, make sure to have a sensible XP scheme and map planning. The Protector TD series makes excellent use of this upgrade type by providing carefully designed levels.

Bubble Tank TD  Upgrade hell of Bubble Tank TD 2, can you understand what is going on here?

Stay tuned for part 2 where I discuss critters and player upgrades (achievements).

Thanks for reading,