Combat is the single broadest area of game design. Some games do not have combat at all, but a large percentage do, and in a large number of varieties. The simplest kind of combat is random combat: flipping a coin, rolling a die, or rock-paper-scissors. This is not satisfying, because even if the player chooses heads or tails (or whatever) that choice doesn't matter. There is no strategy. Even the first generation of primitive games rarely bothered with completely random combat. Instead programmers invented AI, which is programming telling enemies how to react to the player's actions (or changes in their environment). AI is the source of strategic challenge in singleplayer games. That most basic strategy consists of studying enemy AI to be able to predict what you should defend against or how you can manipulate enemies into making themselves vulnerable. (In some cases the player will have one or more AI-controlled ally creatures, which it is also strategically advantageous to be able to predict the behavior of.) Completely predictable AI is also rather boring, because once you solve the initial strategic challenge you don't need to think any more, just repeatedly do what you decided was the most advantageous. So, many AI systems add a moderate amount of randomness to spice things up a bit. This is realistic, because random variation occurs in real life combat (or other activities that AI can direct an NPC unit to participate in) and randomness is certainly easier to add than more complex AI (which wouldn't even have fit in the oldest games due to storage space restrictions on floppy disks and cartridges).
Even today, it is still unknown whether it will ever be possible to create an AI that can simulate realistic human behavior, though some amazing AI programs have been created to accomplish all sorts of goals, from entertaining behavior for robotic toys to word, speech, and face recognition software. Don't be intimidated, though; anyone who knows how to play a game, how to analyze their own behavior, and some basic algebra can design monster AI for a game. It does not require mastery of a programming language to design AI, only to implement that design. The design itself can be done by making a flowchart or a written description of how a unit's decision-making process should work. But, enough about AIs, and back to combat.
The oldest kind of computer game combat is one hit = death (though the player may have multiple lives). This kind of combat originated in text adventure games, but could be seen more recently in many sidescrollers and platformers. The Super Mario series is a well-known example: Mario is traveling toward the right side of the screen, a goomba is traveling left, and whichever one is struck by the other first will die. They don't die simultaneously because they have different vulnerable zones - Mario is vulnerable on his sides, but not beneath him, so he can kill the goomba by landing on top of it. The goomba is vulnerable on its top but not on its sides, so it can kill Mario by walking into him. Of course that's only the most basic situation: Mario may be powered up, and if so the first hit will cause the power-up to fall off instead of instant death. Some monsters are tougher and may need to be hopped on multiple times. A few monsters cannot be killed by hopping on them, but instead must have a turtle shell or other object thrown/slid at them, or a fireball shot at them. The raccoon tail or cape gives him a side attack with a slightly longer range than his vulnerable side area. Etc.
So, as enemies become tougher, it may take several hits to kill them. The monster's toughness is thus called HP (hit points). And then, games where a single mistake kills you tend to be more on the stressful side than the fun side. So we can give the playable avatars and pets HP too, so they won't die from a single hit. It's nice if the player can see how much life they have, so they know when to use health potions or retreat. This is usually shown with a HP meter. There are three standard approaches to visualizing health. The simplest is a bar where the left side represents 0 health and the right side represents maximum health. In some games the maximum health and/or the current health are shown as numbers. The health meter often changes colors from red at low health through yellow or purple at medium health to green or blue at full health. Other color schemes or locations of colors can work too. In some games an injured character has a red nimbus (full-body halo or form-fitting cloud) while a healthy character has a white one, and in other games the whole screen's colors become desaturated (grayed-out) as the character is injured. The second approach is showing health as a clock-shape, where the clock begins filled with color and with a 'hand' pointing upward to 12 o'clock. As the character becomes injured this hand sweeps clockwise, revealing a gray or black of missing health. Again, numbers are optional. The third approach is a heart shape. The heart can either have a vertical meter that shows how full of life the heart is, or can behave like a heart-shaped clock. Horizontal health bars also sometimes include a heart as a graphic indicating that the meter is for health. If numbers are included they are usually written as a fraction: current/max.
Before health meters, landing a direct hit, completely blocking or reflecting a direct hit, and status changes like slowness/quickness, temporary invulnerability, size change, and range decrease/increase were the only things that could happen in combat. The addition of HP enabled DOT (damage over time) attacks and status ailments, such as poison and burn. Not to mention healing (partially or completely refilling an HP meter) and healing over time, commonly known as regen. It also enabled partial blocking and reflecting of damage, and attacks that hurt the user a small amount to hurt the enemy a large amount. Both of these add cost-benefit-analysis strategy to combat, making it more complex and interesting.
Health is far from the only stat playable characters and enemies can have. Magic, energy, and rage are other possible meters that the player might need to manage during combat; in some cases the goal is not to run out, in other cases the goal is not to have too much. Magic, energy, rage, and health are called by a variety of names in different games. There are several other kinds of stats characters and enemies can have which are not typically represented with meters, because they don't go up and down a lot during combat. In turn-based tactical and strategy games these may include action points and movement points, which regenerate at the beginning of each turn and are spent on the character or enemy's actions and movements during that turn. And in RPGs and RPG-influenced genres the character may have long-term stats affected by their equipment, class, stat points that have been spent on that character, and current buffs and debuffs. This group of stats may include defense (armor class), speed (initiative), intelligence, spirit (wisdom), charm (charisma), luck, and stealth.
So from simple platformer combat and turn-based combat the next things that evolved were arcade fighters, which are characterized by the ability of individual moves to add up to combos, and skill cooldowns, which make frequency of use a new factor in combat strategy. Both of these can be found in both turn-based and real-time combat systems. The realtime versions are probably more familiar - cooldowns take the form of a timer, while combos require performing a sequence of actions correctly within a certain time frame without being interrupted by an enemy action or a critical failure. In a turn-based system cooldowns are take the form of a turn number count-down, or more rarely a damage-taken or damage-dealt count-down. Turn-based combos can be executed by two units acting on the same turn (or an enemy and a unit who reflects or counterattacks in response) or they can be executed by one unit taking a set-up action on one turn and then a follow-up action on the next turn.
Another evolution at approximately the same time was the addition of terrain. Technically platformers have terrain, such as spiked ground, slippery surfaces, ground that crumbles when walked on, and underwater areas the playable character must use a different or slower kind of locomotion to travel through. But that's 1D terrain, more or less, and 2D terrain in tactical turn-based combat, strategy combat, FPS, and live-action RPG combat adds more complexity. Distance affects the range of attacks and the speed at which units can pursue or flee each other. In a turn-based game this is usually implemented by giving each unit a number of movement points per turn, while in a real-time game it is usually implemented as a running speed stat. Different kinds of terrain can also give bonuses or penalties to units standing on them or trying to cross. Some terrains types are beneficial to all units or detrimental to all units, while other terrains are sympathetic to some types of units and antagonistic to some types of units (often this is about a unit's elemental affiliation, like fire terrain benefiting fire units and penalizing ice units). Terrains may have obstacles that make them unable to be walked on and may block line-of-sight attacks. Finally, terrain allows for the player to construct fortifications, traps, and other buildings or immobile units, as well as destroy or activate ones which the game has placed there before the beginning of the battle.
So, that's pretty much the whole list of elements that the various modern styles of combat are built out of. Several genres add yet more complexity by blending non-combat elements into combat. RTS games put the infrastructure and workers onto the battlefield where they must be protected from attack. Tactical, RTS, and deck building card game-style combat all may require the player to spend gathered or crafted resources to summon units into play. FPSes and an odd assortment of other games allow avatar(s) to change equipment during combat. Action-adventures may allow adventuring tools to be put to unorthodox use in combat.
What constitutes modern styles of combat? Here is a list. Yes, it's biased. I am deliberately excluding random combat, text-based combat, automated combat, simple turn-based combat as either antiquated or just bad.
- Platformer, Arcade, and Action/Adventure combat: This is a realtime form of combat where the player controls a single unit to perform actions usually bearing some resemblance to martial arts (or things like biting and clawing if the avatar is non-human). Whether armed, unarmed, or able to wield magic in place of/in addition to a weapon, common actions include an upper body attack, a lower body attack, jumping, ducking or rolling, possibly blocking, and possibly a projectile attack or magic attack. Combat takes place in the main/exploring game mode, and the player may end up fighting several enemies at once. During combat the player maneuvers around the screen, trying to place themselves in a position to attack while dodging enemy attacks and also avoiding any holes or other terrain hazards that happen to be present. As with all real-time games, commands must be entered quickly; this limits the avatar's possible actions to a small number of button presses most players can memorize, and limits the amount of strategic thinking the player will have time to do. Some games pause the action to allow the player to select a special action from a menu, but more commonly the player must choose before combat begins which special action(s) to equip to their available button(s) or key(s). Typical features include collision detection, knockback, combos, an obstacle-course-like environment, tool use on both the environment and enemies, and simple status ailments that quickly wear off on their own. Racefighting combat would also go in this category.
- Tactical Turn-based combat: This is a turn-based form of combat where the player controls between one and ten units on a field of terrain. If the player only directly controls one unit they probably can summon AI-controlled allies (e.g. pets) instead. As with all turn-based combat this is slow-paced, and instead of being about adrenaline and resources it is about strategic complexity. Generally the player gets a budget of a set number of movement and action points per unit per turn and each unit has its own stats and set of equipment; in singleplayer versions units may be able to access the player's inventory of consumable items, while in multiplayer games item-use may be unavailable during combat. Combats take place in a limited area with a limited population of enemies, and eliminating all enemies concludes the combat. Common actions include movement across the terrain, ranged attacks which require line of sight, area effect or splash-damage attacks, setting traps, summoning units onto the battlefield, and units healing or buffing team members. Deck building card game combat is a variant of this which usually substitutes some kind of zones and/or creature types for terrain. For example, the player's hand may be one zone and the player's deck another zone, while flying creatures may be assumed to be in a hypothetical sky and non-flying creatures are on the ground.
- Shooter/FPS: This is a realtime form of combat where the player controls only their own avatar, who is generally not visible except as their gun barrel or crosshairs. Common actions consist mainly of sneaking/making use of cover, shooting enemies and picking up found items such as different guns, ammo, and healing items. The emphasis is on reflexes and skill (also luck) rather than stats. Kill combos may be possible (e.g. 3 blue enemies in a row).
- MMORPG: This is a realtime form of combat where the player controls their own avatar, possibly accompanied by 1-3 AI-controlled pets; if there are AI-controlled pets either they or the avatar take a tank role while the other takes a DPS and healing role. Combat takes place in the main world, and it's common for two or more players to fight the same enemy. Enemies are anchored to small territories, which usually prevents more than two enemies from attacking the same player, and enemies will 'leash' back to this territory if the player flees. Enemy AI is usually less complex than in a tactical/strategic system; this is a side effect of class balancing more than a result of the simplicity required of all realtime combat. Typical features include timed skill cooldowns, aggro (enemies preferentially attack whichever unit they are the most mad at), a party system with automatic or configurable loot division, spellbars, a basic attack that repeats automatically, classes with different combat styles and equipment (this is not necessarily a GOOD feature), and player-alterable stats/purchasable skills.
- [Combat Type]
[indent=1]- [Are enemies visible before combat begins, and is combat in the main game mode or its own separate mode? What non-combat activities, such as resource gathering and constructing buildings, can or must be done during combat?]
[indent=1]- [What unit(s) is the player controlling in the fight and what can the player do with them, including movement and types of attacks?]
[indent=1]- [Are there AI allies of some kind?]
[indent=1]- [What attributes do units have? Equipment, an equipped/learned subset of available abilities, what types of stats from HP to AP and Movement/Speed, or elemental attributes, and are these stats player-alterable...?]
[indent=1]- [What is the battlefield like and in what ways can it effect combat?]
[indent=1]- [Can items be used in combat, and if so, how?]
[indent=1]- [What is the typical number of enemies fought at once, as well as the minimum and maximum number?]
[indent=1]- [Are enemies typically different in stats or other capabilities from player-controlled units?]
[indent=1]- [What is typical enemy behavior and common variants on that?]
[indent=1]- [What are the typical rewards of combat?]
