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Wednesday, November 23, 2011

Traps and Triggers

There's a thread on Dragonsfoot about using the old school approach to detecting and disabling traps; in other words, having players describe how they search for traps and what they do to disable or avoid triggering the trap. The person who started the thread wasn't sure how to handle this: what possible ways could a player use to search? How do you enable people with no special knowledge of traps in the real world to role-play a knowledgeable character in a fantasy world? What if the player just creates a huge list of actions to search for traps and says "I do all this" every time? I responded in thread mainly to the last point, but it occurred to me that I could do a whole series of posts on the topic, based not on any special knowledge, but just thinking about basic mechanics like levers and pulleys, what they could do, where they could be located, and how they would be noticed.

The important things to remember are:
  • A trap has two parts: the trigger and the result.
  • Each part must be detected separately, and may require different search methods.
  • Search order is important; one method may find a trigger, another method may trigger it.
There are a couple very basic triggers, but these can be chained (the result of Trigger A is the triggering of Trigger B.) They can also work in parallel (a lever in Trigger A moves two other levers simultaneously, triggering Triggers B and C.) Sometimes Trigger A disengages Trigger B, making another action safe, so finding a "trap" and avoiding or disabling it is not always a good thing.

Results can be pretty varied, but they can all be simplified to an action delivered along a particular path, from Origin Point A to Target Point B. A character standing at the target point or along the path between the origin and the target is in danger of being caught in the trap. Examining Point A may locate a trap, but not its trigger; examining Point B may offer clues to what kind of result to expect (scorch marks on a wall are a clue to a flame thrower trap, debris on the floor may be a clue to a deadfall.) Some results, like releasing a gas, might not be obvious from any kind of clue.

When constructing a trap, you should consider both obvious actions (walking over a trapped floor, opening a trapped door) and search methods (touching or moving a trapped chest.) Sometimes, a particular search method will trigger the trap, which might not be a good idea based on whether you are standing in the path of the result or not.
Consider this simple setup: you have two parallel corridors connecting two locations. In each corridor, there are two adjacent pressure plate triggers, each one the width of the entire corridor, so that people walking down the corridor first step on one trigger, then the other. There is no normal way to jump over the triggers (the distance is too great,) but magic could avoid them. One trigger disengages the second trigger in that corridor while pressure is applied and for one round after it is released; the second trigger, if pressure is applied when it is not disengaged, causes something bad to happen.

The result of this setup is that walking normally down the corridor in one direction is safe, while walking in the opposite direction triggers the bad result. You now have a one-way corridor. The parallel corridor has the same setup, but with the opposite direction; the inhabitants of the dungeon know to use the left-hand corridor, whichever direction they are headed (the parallel corridors could, of course, be merged, and the inhabitants would walk single-file along the left-hand side, much like road traffic in the U.S.)

A visual search of a "one-way corridor" could find both triggers, but that doesn't help the adventurers figure out that what matters is the order the triggers are stepped on. Using a 10-foot pole to press Trigger A (which disengages the trap) does nothing visible; pressing Trigger B within one round of pressing Trigger A will also do nothing visible, since the trap is disengaged. At this point, a party of adventurers may decide there's no danger and continue down the corridor, which might have no effect if they are heading the right direction. It's when the now-inattentive party returns down the same corridor when the problems begin.

I've got more specific things to say about each kind of trigger later.


  1. One recurring problem I see with dungeon traps is that often neither the player nor the DM really knows how the trap would work, so the whole problem space is abstract - there's a doodad - lever, wire, some other thing that says "trigger" but nothing else - and I have a choice to mess with it or not. IME this can lead to "darkness paralysis", especially if you've ever played a trick on the players where doing the less expected thing leads to their doom.

    Actually, if the trap leaves visible traces in the environment, then it's probably not "plumbed in," like those falling stone doors that fictional pyramid designers are addicted to - it's more improvised, standing on the surface. So if you find a wire you should be able to trace it to some device. Traps that are plumbed in... I personally think might best be handled with the players having some sort of advance warning that allows them to treat the environment as a puzzle. Maybe they have fragments of a previous adventurer's journal, or a manifest of parts that the dungeon-builder ordered, or something that can clue them in to what's ahead.

    If you actually do want your players to slow to a crawl and exhibit the kind of paranoia that turns every crack into a potential poison gas vent then of course you need to get very smart about how you signal at the table those things in the environment that might be threatening. That seems like the kind of thing -C writes about.

    /unsolicited nosing. I'm looking forward to your series of posts.

  2. This is an interesting topic, but a problem that I have never seen come up in games that I run, because of how I referee traps. Unless the trap mechanism is very crude (something like a bucket placed precariously on an ajar door or a darts that fire out of holes whenever black flagstones are walked on) I handle the search relatively abstractly.

    For example, if there is a poison needle trap in a treasure chest's locking mechanism (surely a quite sophisticated machine, when you think about it) a player indicating that they inspect the lock will be enough to detect it.