This treatise--well, blog--will articulate the main ideas behind tower defense computer games. This should be handy to software developers as they write the games as well as to players as they create or hone their strategy and tactics. Normal game play is not discussed, as it is assumed that any reader of this blog will already know the basics of tower defense games. Throughout the blog are links to online flash games that you can play now!
General description: A tower defense (TD) game is a computer strategy game in which the player attempts to defend his* or her castle, by building various types of attack towers, from attacking waves of creeps which are approaching via a determined path.
*I will usually be using "his" and "he", as TD players tend to be male. This in no way should discourage female TD players!
Components.
The Board.
- Maze. In a maze-board, the creeps attack along a path pre-determined and generally unchanging. Some TD games allow the player to select from several mazes. The straighter the maze, the more difficult the game. (A game that is good, but has a frustratingly short maze is Ravine.
- Open. In an open board, the player must build the path by placing towers in such a manner as to block the creeps from going any way except the desired path.
- Two-dimensional. These games are simple cousins of TD's and will not be specifically covered in this blog, though the general principles of TD games will apply to two-dimensional TDs.
- All boards have three main components: entry point(s), the path, and the castle.
- Most games only have one castle.
- Some games will allow you to upgrade your castle: greater defense, stronger walls, more HP, special powers, etc.
- On open-board TDs, the castle is usually accessible from many directions (until the player builds a path.)
- On maze-board TDs, the castle is usually accessible from only one or two directions.
- A few TDs have "invisible" castles. This can be merely that creeps need to get to the other side of the board, or at least one game ends as soon as 100 creeps are on the screen. [Desktop Tower Defense (now Desktop TD Pro)is a classic example of this type.]
- Once built, towers are never movable.
- Towers are usually upgradable to inflict higher damage, have greater range, fire at a faster rate, or have special powers. Each level in generally increasingly expensive.
- Towers can usually be sold. Prices vary widely, but one-half cost is a good average.
- Some games allow the player to prioritize the tower's targeting. So, players can choose to fire at the weakest, the strongest, the first, the last, the closest to the tower, the fastest, etc.
- The damage done by towers is frequently an exact amount, though sometimes they have a range of damage.
- Every game has different types of towers (covered below), each of which will have a certain range of attack.
- Creeps attack in waves, either on a timer or after the player clicks for the next wave.
- Each wave has increasingly difficult creeps.
- Creeps are sometimes able to fire at your towers.
- Each creep type (see below) has specific strengths & weaknesses. Oftentimes, waves consist of only one type of creep.
- Creeps always take the shortest route to the castle. Sometimes a little randomness is mixed in with the creeps' direction, but the shortest route is still key. [The best example of this random element is the fun game Antbuster, in which you are not allowed to build a maze, since there is so much randomness built into the creeps' movement.
- Creeps may have several entry points on an open-board TD. Often, however, even open-board TDs have just one or two "doorways" through which the creeps enter.
[Note: Here I will be using nomenclature that is partially industry-standard, and partially based on that determined by the best TD games. The descriptions of each type of tower is based on the average TD game; most TDs will have variations, of course, but these are the basic types most frequently encountered.]
- Arrow: inexpensive, fast rate of fire, low damage, medium attack range, attacks ground & air units.
- Cannon: moderately expensive, slow rate of fire, high damage, sometimes splash damage, long attack range, ground units only.
- Air Attack: moderately expensive, medium rate of fire, medium to high damage, long attack range, air units only.
- Slow: expensive, medium rate of fire, low damage, small attack range, attacks ground & air units, slows and/or freezes units upon hit for a limited amount of time. Sometimes it will have a splash effect.
- Poison: expensive, medium rate of fire, low damage that continues for a limited amount of time, medium attack range, attacks ground & air units.
- Splash: a hit which affects multiple creeps at once (usually the creeps closest to the center receive the most damage.) This is either a stand-alone tower, or an effect of other towers, such as cannon, slow, and poison. Sometimes towers are upgradable to include splash damage.
- Multiple: a tower which attacks more than one target, usually randomly. Moderately expensive, medium rate of fire, usually a low attack range, attacks ground & air creeps. Usually a stand-alone tower, but sometimes towers (especially arrow and air) are upgradable to include multiple attacks.
- Investment/research: these towers do not attack but gain extra money/mana. Sometimes they are a prerequisite for building other types of towers.
- Magic: a tower type sometimes used. Usually expensive, medium rate of fire, high damage, long attack range, attacks ground & air units. Sometimes used just to make things harder when creeps immune to magic (see below) show up!
[Note: the same note for Tower Types applies here. Further note: creeps and towers within a specific TD game are always complementary.]
- Creeps either creep on the ground or fly in the air. The flyers are still creeps, though!
- Normal: medium on everything, no immunities.
- Slow/strong: high HP, slow speed, heavy armor.
- Fast/weak: low HP, high speed, light armor.
- Immunities: These creeps are usually one of the above three types, but with immunity to one of your tower types: esp., immune to poison, slow/freeze, or magic.
- Boss: insanely high HP, medium speed, heavy armor. Usually alone, sometimes paired with weaker creeps.)
[Note: see note to Tower Types. Not all TD games use traps; indeed, many fine ones do not.]
Traps are placed on the creep path. They affect all creeps that walk (or sometimes fly) over them. Trap features vary, but they are sometimes permament & sometimes wear out; they are sometimes static and sometimes upgradable. Ones that wear out are usually low cost, ones that are permament and/or upgradable usually cost about the same as a tower. The types of traps are similar to the towers:
- Damage: usually a low amount, sometimes with a splash damage, sometimes magic.
- Slow/freeze.
- Poison: this will be less effective than a poison tower, but more effective in that it poisons every creep.
- Wall: this will always wear out eventually, but is usually upgradable.
- Reduce immunity: reduces creeps' immunity by a small amount. Many in a row can be very effective.
- Increase money/mana: a small increase per creep that passes. Obviously to be placed as early along the creep path as possible.
A Hero is a single entity that the player controls with (usually) the mouse. Their presence gives an arcade feel to the TD game, which contrasts with the strategy genre. Although none of my favorite tower defense games have a hero, they are sometimes entertaining for a while.
- The Hero is usually able to move anywhere on the board.
- The Hero always targets whatever creep the player wants. This, in fact, is the whole point of the Hero, as far as I can see. TD games that allow the player to prioritize the towers' targets, Heroes are superfluous.
- The Hero is usually upgradable through experience or money.
- The Hero usually has health to beware of.
- In two-dimensional TDs, the Hero is usually present. In some cases, the "Hero" is just a cross-hair that the player uses to target the creeps.
- The player usually starts off with enough money to buy two or three towers.
- Money is almost always gained per creep killed.
- Money is usually gained per creep wave passed.
- Sometimes interest is earned. Players should take note of when interest is calculated and be sure to make big purchases after the interest has been earned.
- Tower choices appear on a side panel.
- Players can usually buy/upgrade towers during a creep wave, as well as between waves.
- There is either a wave timer or a "send next wave now" button.
- Some TD games display what types of creeps are coming in the next wave(s).
- Points, money, etc. will constantly be displayed.
- The board. Sometimes this is scrollable or zoomable.
- Clicking on a tower will select it for sale, upgrade, or targeting.
- Gives the player numbers: how much damage a tower does, how many HP creeps have.
- Upgradable towers.
- At least 5-6 types of towers.
- Alternate challenge such as: earning money, researching technology, special attacks, complexity of creep/tower weaknesses & strengths.
- Easy tower build/upgrade interface.
- “Speed up” ability to bring on waves sooner and/or increase play speed.
- Interest or bonus for meeting certain performance criteria (money at end of level, how close to beginning you kill the creep, awards a la Gemcraft, etc.)
- Must not unduly slow down during high levels. This becomes one of the most important issues when deciding whether or not to replay an otherwise good game. Keep the graphics simple: we are not playing TD's for nifty graphics, we are playing for the strategy & challenge. (Cute graphics can be fun, however. Mushroom Revolution is a good example of this!)
- Upgrading a tower is usually better than building a new one.
- Path should have at least two strong attack points: one at beginning, and one half-way through.
- Open-board paths should do two things: maximize creep path, and bring creeps repeatedly within range of strongest towers.
- A good strategy with open-board TDs can be to build the path with a bunch of arrow towers. Once the path is established, the arrow towers can be replaced with stronger tower types. This strategy is largely dependent upon the resale value of towers.
- Save as much money as possible, especially if interest is earned. Win each wave with maximum effort exerted.
- If interest is earned, be sure to buy/upgrade towers after interest for that wave has been calculated--the longer you hold off buying/upgrading the tower, the more interest you've earned.
- If research or prerequisites or general improvements (i.e., money or mana increase) are allowed, invest in those as early as possible to reap maximum benefit. [A good example of this is Bloons 4. In this TD, you'll want to buy (and upgrade!) the banana farm as soon as you possibly can.
- A variety of towers is essential. Early on, you will want at least one cannon, and one air tower, along with a few arrow towers. [For some rreason, the silly & comic game Fratboy Girlfriend Defense is the best example of each type of tower being needed in order to battle a specific type of creep.]
- Air creeps will often be the bane of your existence. Since they don't stick to the path, air creeps must be killed quickly. This becomes very difficult once it takes several hits from a fully upgraded air tower to kill a single creep.
- Some mazes will be different and you may not even need certain types of towers: especially slow, magic, or random multiple-attack.
- If poison towers are available, set them up periodically, so that the creeps are constantly poisoned. No need to have three poison towers next to each other, unless they fire quite slowly.
- If you are playing an open-board game, try building two (or more) mazes. Once the creeps get near the end of one maze, close it off and open up the other route. Now the creeps have to go all the way back through your deadly maze to get to the new exit. This is an essential strategy for high scores in some games like WarZone. [Beware of WarZone because even though it's highly addictive, the game play is slow and a single game can last a looooong time.]
Author's note: Of all the free online tower defense games I have ever played (and I have played all that I can find), the best is Gemcraft (and now also Gemcraft Chapter 0). This combines ease of play, complexity of powers, variety of mazes, minor rpg storyline, and a looong time playing before completing the gam
Tower Defense Games, alphabetically, rated (A,B,C,D,F) & described:
- Born of Fire TD Grade B+. A "low-budget" version of more complex & lengthy TDs, this is still a good one that mixes RPG in. Instead of towers, you have "heroes" who are recurring towers that level up over time.
- Chaotic Perim Protector. Grade C. An advertising gimmick for what appears to be a playing card company. Still, it's fine with interesting cartoony graphics. Could be good with added features such as creep health, targeting, different mazes, etc.
- Civilizations Wars. Grade A. Towers are in fixed positions on the board: you need to direct your men to take them over. Each tower you capture has abilities: generate soldiers, generate mana, arrow defense. You are, of course, competing against other players attempting to do the same. First person to control the board wins. Very well done.
- Colony. Grade B. This is good, complex. Combines two-dimensional combat with tower building. Very well done, probably a Grade A if you like 2D.Crimson Warfare. Grade: D. Two-dimensional/linear TD. Tedious with no redeeming qualities, though no totally fatal flaws. Would benefit greatly with automatic build feature.
- Crop Circles 2. Grade: C. Cute idea of crop circle creep path with aliens coming to eat farmers' brains. Simple & relatively easy. Has the uncommon feature of allowing you to lay out the creep path first, then place your towers as you gain money.
- Defend Your Honor. Grade B. Cartoony graphics, introductory cartoon, intermission scenes of doors you go through to pick your level. Effort went into this, it is a simple TD in both good & mediocre ways.
- Desktop TD. Grade A. A classic tower defense game that has two lines of creeps going to two different exits. An open board game which has the only flaw of a small playing field. One tricky part of this game is that each time you sell a tower, it takes longer & longer for that tower to disappear! (Note: several versions out, but I like 1.5 best.)
- Fratboy Girlfriend Tower Defense. Grade A. Stop the creeps from getting your girlfriend! A winnable TD game with a few slightly humorous interludes. Simple but well done. I actually think this is the first TD I ever played & look where it got me! :)
- Gemcraft (& Gemcraft Chapter 0). Grade A+. See above. These are the best tower defense games I have yet discovered.
- Ghost Hacker. Grade B+. Simple, but interesting. Upgrades are limited so more strategy is required.
- Highlord TD. Grade B. An RPG game, with large map. Different feature is that you have four little guys that you release from your tower--each guy does a different thing to the enemy tower. Round ends when one of these 4 guys destroys the tower. Towers upgradable with experience only. A little boring, a few minor bugs: a bit more work would make this an excellent game.
- Inner City TD. Grade F. This one fails, so no link! Conceptually just fine, but poorly executed. No challenge, no real interest. Sound did not work. Won the game on the first try.
- Paper War. Grade A. Interesting feature of this one is that creep path is similar to roller coaster: as creeps go "down" they accelerate, and as they go up they decelerate. Relatively simple & well executed.
- PC.DEfense. Grade A-. Excellent though simple tower defense. For some reason, the simplicity of this game works.
- Pirate Defense. Grade A-. The 'hook' to this one is that you build traps rather than towers (well, there's one kind of tower.) Well done, though fairly easy. Slows down significantly with lots of creeps. Creeps are not very smart (or maybe, too smart) and you can trick them with a loop that offers an "easy" way and a "hard" way--they'll get stuck.
- Renegade Commanders. Grade B. Build up your army base to destroy the enemy's! Great graphics, gameplay is in 3-D. Open board, not a true TD game, but a military strategy game. However, using basic TD principles can enhance your game!
- Savior TD. Grade B. Another good game from ArmorGames.com, this one features good medieval graphics and some cool towers on a maze path. Main drawback is slow game play--the creeps really do creep along, and there is no option to speed it up. Pretty basic, some kind of story line, no real "hooks" but no real flaws, either. If you have the time to wait for the creeps to move along, this is would be a Grade A.
- Temple Guardian 2. Grade B. Defend your temple while earning income, researching new weapons & acquiring a hero. Complex, scrolling map, hero, pseudo-RPG, enemies attack towers, research required, real-time (and good soundtrack!). Overall very good, but lacks a little on playability: screen is too small & requires too much scrolling & game play is fast, reducing importance of strategy.
- The Tower & the Heart in Glass. Grade A. A two-dimensional/linear tower defence game. One of the best of its type. One interesting thing about this game is that you can place the "heart in glass" (that which you are trying to defend) anywhere you like.
- Tower of Doom. Grade C+. Kind of fun, but lacks any strategy in tower placement or creep path design. An interesting concept, though, with good graphics and music. You basically have one giant tower in the center of an open board, and the creeps come from all directions. Sort of phallic, as when you add "new" towers, you are really just building a floor onto your tower, so it gets bigger & bigger!
Copyright December 29, 2009 by Scott Givens. This will be updated very soon with lots & lots of links to various online TD games. Online tower defense video games can be very addicting, so beware!
