A taxonomy of traps

 Traps have always been a staple of dungeon-delving adventures. There's more than one way to classify traps, but here I'll be looking at them according to their purpose in the dungeon, both in-fiction (i.e. the purpose they serve to their creators in the game world) and in-game (i.e. what they add to the experience for DMs and players.) Of course, there is a fair bit of overlap, and any given trap might fit into more than one general category. 

HAZARD: Not truly a trap, per se, but some feature of a dungeon that presents a danger to explorers. Unlike a true trap, a hazard is not intentionally constructed, but functions in the game in a similar way. Hazards may be natural features, such as a sinkhole, an unstable pile of rubble, a pocket of toxic volcanic gas, or the result of deteriorating structures, such as a crumbling bridge or loose ceiling block.

    Purpose: In the fiction, being products of impersonal forces, they have no purpose per se. In a game sense, they keep players on their toes and add verisimilitude to a dungeon. 
    The danger: Varies widely. Depending on the specifics, they could result in death, incapacitation, confinement, or alerting nearby monsters.

OBSTACLE: This sort of trap is usually very obvious and easy to spot, but presents a challenge to get past. An open pit, a constantly swinging pendulum blade, or a corridor in which the walls repeatedly slam together and separate again are examples. Many of the traps found in Ayleid ruins in the video game The Elder Scrolls: Oblivion are of this general type. 

     Purpose: Intelligent dungeon denizens may construct this kind of trap to slow and frustrate intruders, and to give them an advantage when pursuing or fleeing from said intruders. This kind of trap can maim or kill, but the risk is generally minimal to reasonably observant and intelligent players. They can pose a delay or consume resources to defeat them. They can be potentially deadly when time or lack of light does not permit proper caution, such as when fleeing from or chasing enemies.

    The danger: Varies from deadly to minor inconvenience. 

ALARM: A trap which alerts someone to the presence of intruders. It typically will make either make noise at the site of the trigger or transmit the alert to another location such as a guard post, with seemingly nothing happening at the trap site. 

     Purpose: Alerts dungeon dwellers to the presence of intruders. In game terms, triggering an alarm forces the players to think and react quickly. In an active stronghold or lair, a guard contingent will likely arrive soon. Otherwise, it may require a wandering monster check, or at least put a scare into the players.

    The danger: It's generally not good to have monsters showing up while you're exploring. The exact danger depends on the kind and disposition of the monsters.

CONFINEMENT: Confining traps will not directly harm characters (at least not very much) but prevent them from escaping. A pit with a lid that closes again, a portcullis dropping over an exit, doors that automatically lock, or a falling net are common examples. Lesser examples might catch single characters, such as a sinking block that catches a foot in the floor and leg snares that hoist a victim upside-down by one leg (which also has some elements of an Incapacitation-type trap, in that it hinders the victim's actions more than simple confinement.)

    Purpose: In-fiction, these are usually used by creatures or people who wish to take their quarry alive, either to capture or keep it from going somewhere they'd rather it not go. For the players, it poses a challenge to overcome or escape without immediately threatening the lives of their characters. 

    The danger: Time and resources may be consumed in escaping the trap. It's not good to be confined when a guard patrol or wandering monster happens along. If the trap is combined with an Alarm, this may happen quite soon; otherwise, it may be a while. If the characters can't escape on their own (especially dangerous when adventuring alone!) and nobody comes along, they could die of thirst or starvation.

DETERRENT: Like the Obstacle, above, a deterrent trap is easily detected. In fact, it's meant to be seen. Unlike a mere obstacle, though, it's usually meant to be deadly, even gruesome, and not easily disarmed or avoided. It says in no uncertain terms that intruders are not welcome. 

    Purpose: Diegetically, exactly what it says on the tin, a No Trespassing sign with really big nasty teeth. Game-wise, it's a temptation to the players, as it telegraphs both the likely presence of something of great value or importance and the horrific fate that may befall them if they decide to meddle with it.

    The danger: Potentially lethal (and messily so!) but entirely at the discretion of the players. They can either accept the risks of tackling it, or pass it by, letting it serve its intended purpose of deterrence. 

PUNISHMENT: Similar to the Deterrent, but non-obvious, perhaps very difficult to spot, this type of trap inflicts damage or death on anyone who meddles with the object it protects. The classic poison needle in the lock of a treasure chest is a good example. While one of the most well-known trap archetypes, it's one of the most, well, punishing, and ought to be used sparingly.

    Purpose: Simply to prevent anyone from doing something by inflicting harsh consequences. It's very difficult to unlock a chest and make off with the treasure if you're dead. In game terms, this keeps players from being complacent, because they never know when an object might be protected by such a trap. 

    The danger: Usually death. Sometimes the trap inflicts damage rather than a save-or-die effect, but a character not sufficiently tough could still die.

INCAPACITATION: An incapacitating trap is similar to a confining one, but instead of physically containing victims, it knocks them out, puts them to sleep, stuns them, paralyzes them, or otherwise renders them helpless. Such effects can be accomplished by magic, liquid or gaseous toxins, or mundane means like leg snares, gluing victims' feet to the floor, or burying them to their armpits in sand.

    Purpose: In fiction, these serve similar purposes to the Confinement-type trap, allowing dungeon denizens to deal with intruders without necessarily killing them.

    The danger: As long as the effect lasts, victims are extremely vulnerable to any monsters that happen by, whether random or organized patrols. And while the builder of the trap may intend to take the victims alive, a random monster might just as soon kill and eat them.

WEAKENING: The trap is designed to wear down intruders and/or deplete their resources. It's seldom lethal in and of itself, but can leave a party at less than full strength before a major confrontation. Darts (non-poisoned), gouts of flame, or numbing gas would all qualify. Sometimes another type of trap can double as this kind, such as a 10' deep pit that's both an obstacle or confinement type, and does a few points of damage.

    Purpose: In the fiction, this kind of trap might be used by dungeon dwellers who want to discourage adventurers from exploring further without actually killing them. Perhaps they don't want to incur the level of wrath that lethal traps would raise, or maybe they lack the resources or the cleverness to build really deadly stuff. Game-wise, this kind of trap is a great reminder that the dungeon is a dangerous place without being too heavy-handed on the players.

    The danger: A few lost hit points, using up a spell or two, or losing a bit of equipment. The real risk is in squandering those resources so they aren't available when the stakes are higher.

PUZZLE: A trap that, by design, may be circumvented simply by knowing or deducing the proper solution. A failed attempt usually results in the trap being sprung, triggering an effect that may be deadly, damaging, confining, incapacitating, or weakening. It may involve solving a riddle or possessing some knowledge which enables one to press the right buttons or pull the right levers to disable the trap, or to take some other action to avoid and bypass it. Think of the traps in Indiana Jones and the Last Crusade, for example: the clue "Only the penitent man shall pass" is a hint that one must kneel to avoid being decapitated by a sweeping blade.

    Purpose: This type of trap is generally made to let the "right" people in while keeping the "wrong" ones out. For the players, it's an opportunity to succeed by tackling a good old-fashioned brain teaser.

    The danger: Varies widely, as noted above.

These are the ones that spring readily to my mind, but it's possible I've overlooked something. If so, let me know in the comments!