Traps are an important part of dungeon fantasy gaming.
GURPS Dungeon Fantasy 2: Dungeons, p. 19-20 outlines clearly how DF handles traps. It's the usual stuff you need to know (or just end up needing to know):
- Detect - how hard to detect?
- Disarm - how hard to disarm?
- Circumvent - how hard to get around?
- Evade - how hard to evade it after it goes off?
- Effects - what happens?
- Shots, Rearm, Steal - How many shots, can it be rearmed, can you steal some or all of it?
Here is roughly how I handle the first five.
Douglas Cole has often said, "If the answer isn't the Speed/Range Table, you're not asking the right question." For me, though, I often think the Task Difficulty rules are the way to go. They're on p. 345-346 of GURPS Basic Set, and then range from +10 to -10, centered on a normal use at 0.
+10 is for a task so trivial you probably shouldn't require a roll unless there are other circumstances involved.
0 if for average adventuring tasks - the example is a Driving roll in a car chase (and a high speed car chase is only -1).
-10 is impossible, or nearly so.
You can run a whole game with nothing but this kind of range in hand - in fact, one of my favorites, Yaquinto's Pirates & Plunder, did just that.
I'm also a big fan of the ". . . With Spikes" ruling in DF that says that every horrible qualifier attached to a situation is a further -1 (or potentially, a +1). Extending that means walking on a slick, narrow ledge is -2, but walking on a wide, coarse ledge is +2. I extend that right into traps in at least one place.
So with that ruling and the Task Difficulty rules in mind, here is what I do.
Ask, how hard is it to detect?
Will the normal Per-based Traps roll to spot it find it easily (as much as +10 to see it for being in plain sight) all the way down to nearly impossible to spot (-10 for fiendishly well hidden). In the middle is are ones you expect a fairly average skill to have a reasonable shot of detecting.
Is it invisible and requires special detection? If the trigger is magical and you can't detect magic (no Magery or Mage Sight or Mage Sense), or invisible and you lack See Invisible, you might not even get a roll. Magical traps especially can call for Per-based Thaumatology or Per+Magery, because there is no non-magical component to see. These are trivial for wizards to detect and are just Wandering Damage for those lacking Magery. Moral of the story: have a Scout with Magery or throw the right spells on him.
Think about both the trigger, and the trap. They might end up with separate numbers in a complex trap. It might be very easy to see the trap, but fiendishly hard to see what triggers it.
You want to keep in mind that by default GURPS DF parties are looking for traps, and get a Vision-based Per-based Traps roll to see a trap. The base Thief and base Scout both have a 15 or less to spot a trap. So they spot easy traps on a 25 or less (i.e. automatically) all the way down to a 5 or less to spot a really well-concealed trap (the example crossbow trap in DF2 is -9 to spot). The others, no so much. A Knight with Per 10 and default traps has a 5, so he'll spot a pit on a 15 or less, while his Wizard and Barbarian buddies have a default of 7 and spot one on a 17 or less.
In any case, they don't even need to ask for these rolls, because the default movement mode is slow enough to allow this roll and assumes you're on the lookout for danger. Remember, though is is -5 when rushed, so only trivially detectable traps are going to be automatically spotted by them, and what's "average" difficulty to spot is going to be missed half the time! Moral is, don't run through the trapped dungeon.
How hard is it to disarm? I use the same range of penalties as above, from "trivial" and +10 up to nearly impossible and -10.
I consider two parts of the trap for disarming:
How had is it to disarm the trap itself? Does it require some trivial expenditure of resources (break a poisoned needle, stuff up the dart launcher holes, erase the magic runes? Does it require some complex skill to disarm?
If the process is complex, give it a high penalty, up to a -10. If it's easy, up to +10.
In any case, you need to know if a failed disarm sets it off.
Some of the issue with disarming might be purely mechanical. It might be impossible to disarm a big rolling boulder or a deadfall - you can't just spike them in place or bend them back with a dagger. Disarm on the trap itself might be "N/A." The best way to deal with such traps is to avoid them entirely, set them off safely (think of a bomb squad here), or disarm the trigger.
If stopping the trap from going off is all that's important, disarm the trigger.
One thing to consider is, how does the trigger work? If the trigger is your basic deadman switch, it's hard to render it harmless without potentially setting off the trap. If it's a tripwire or pressure plate that depends on mechanical pressure to set it off, cutting it/lodging it in place might be enough to stop it from working. This is largely going to be a straight-up DX-based Traps roll, or, if the fix is merely go all Alexander-and-the-Gordion-Knot on it, just smacking it and seeing if it goes off.
In any case, you need to know if a failed disarm here too sets it off.
For a whole series on trap and triggers, take a look at The Nine and Thirty Kingdoms, especially down a few posts.
Circumvent / Evade
Or as I put it, Avoid. How hard is this to avoid? Both Circumvent and Evade have related questions - how do I get past it without setting it off, and how do I avoid getting hurt if I do set it off?
The first is best deal with using the Task Difficulty modifiers or the ". . . With Spikes" ruling, by treating it as an obstacle. You need to know how hard it is to pick your way around the trap triggers (think Raiders of the Lost Ark, avoiding the blowgun dart triggers). In many cases traps will be combined with channeled movement to make it harder to avoid even if you know it's there.
The second is where Resistance Rolls, Dodge, etc. comes in. It's an attack and it's incoming, how do you avoid it? Penalties should be applied like any other attack. Magic effects should get a resistance roll (if appropriate), weapon-like effects a Dodge or Block or even Parry, poisons a resistance or avoidance roll, etc. I don't use the Task Difficulty numbers here because a -10 to resist or -5 to Dodge means a whole different thing that -10 to detect or -5 to disarm.
How much damage does this do?
Here I skip the Task Difficulty rules because they don't apply.
What I do instead is consider the range of damage as a minimum, average or mean, and maximum. That helps determine its lethality. I put them in three ranges, which roughly correspond to "Fodder, Worthy, or Boss."
Mostly Harmless: If the maximum damage of a trap is going to put 0 damage past most of the DR of the group as a max, it's mostly harmless. For example, a shuriken launcher that sprays a dozen shuriken doing 1d-2 cutting is mostly harmless against a group sporting mostly DR 4+. It's possible a high damage roll or two against a low DR hit location might happen, but the odds are it'll do 0 damage. If it can inflict actual damage, like say a 6' pit (2 yards, 1d+1), but not enough to threaten a major wound, it's still Mostly Harmless. Unless you fall on your hand or foot and roll maximum damage, it's not going to slow you down.
Non-damaging effects with a high resistance bonus, low skill, or minimally disruptive effects (you're sprayed with paint, you're at a -1 at most on some or all stats, etc.) are Mostly Harmless.
Dangerous: If the average/mean damage of a trap is going to put some significant damage past the average DR of the group, the trap is dangerous. Now it matters. A 15' pit (5 yards, 2d) with spikes does 2-12 impaling damage, average 7. Against that same DR 4 average, that's 6 damage to whoever falls in. Not a lot, but it's the same as getting hit with a good whack in combat and it'll require a potion or spell to get rid of. On a normal person (HP 10), it's a major wound. A crossbow trap doing 1d+5 impaling (6-11, average 10.5) is similarly dangerous. Basically, if there is a threat of a major wound or crippled limb or extremity, it's dangerous.
Non-damaging effects that can potentially kill, or inflict a non-lethal but annoying effect, and/or which have a normal resistance (spell roll of 15, straight-up HT rolls, etc.) are Dangerous.
Lethal: If the minimum damage of a trap is going to inflict a crippled limb or major wound, the trap potentially lethal. If the maximum damage can kill outright (or at least force 1+ death checks), it's potentially lethal.
Something that's causing 10+ net damage is very serious; 20+ is potentially lethal. 50-60+ is a serious threat of lethality. The upper end of lethality in DF2 is a 100 yard pit doing 9d+1 (10-73, average 32.5) or lava at 8d+2 burning damage per second. You might not die immediately, but dying is coming at you quickly.
Non-damage effects that can easily kill belong in this category. Drowning, suffocation, the incapacitating afflictions, etc. are all potentially lethal.
These are pretty broad categories, but that's because so many factors impinge on them. DR of the party members, HP of the person hit, resistance rolls, effects, special circumstances, etc. Even a Mostly Harmless trap can turn out to be lethal if it's combined with a monster encounter that takes advantage of the trap (you're dealing with avoiding falling in dozens of little pits that slow you down while the monster kills from afar, say). A trap that is Mostly Harmless to the 20 HP DR 10 knight might be Lethal to the 10 HP 2 DR Wizard. "Only 12 impaling? Okay, 2 gets through and I take 4 injury, I'm down to 16 but bandaging will get me back to 18." vs. "12 impaling? I'm at -20 and I need to make a death check." The net effect of the circumstances matter.
Putting it together
You need to look at the sum of the parts. The teleport over water full of razor fish in my game was easy to detect (but not blatantly trap), hard to disarm, and potentially lethal = very dangerous. The water itself was easy to detect, easy to avoid, and potentially lethal. So all in all it was dangerous but unlikely to TPK the group - yet it almost did because they pushed their luck too far. All in all, that was a potentially lethal trap.
Some other traps have been trivially dealt with, because for all of their damage they were easy to detect, easy to disarm, or easy to set off with no harm to the PCs. Most of the ones set off recently in my game with Create Servant were like that - a more recent linked "bear trap on a deadman switch for a deadfall" one was potentially dangerous (category 2, above) but easy to deal with.
Consider the parts and then the sum of the parts, and see how dangerous that looks to be. How likely are they to blunder into it, get out of it, and/or survive it? Remember DF delvers aren't fragile, but aren't supermen either. You don't need traps to be hard to detect, hard to disarm, impossible to circumvent, difficult to resist, and highly damaging all at once.
Finally, I like stat lines, so I'll boil it down in my adventure descriptions to things like this:
10 x 10 x 10' Spiked Pit (Detect +10, Disarm N/A, Avoid -2 (precarious narrow ledges), 1d+2 imp)
Poisoned Needle (Detect -3, Disarm -1, Avoid HT-2, 4d/2d toxic no delay no cycles)
Lightning Runes (Detect Per+Magery or Per-based Thaumotology, Disarm N/A, Avoid DX-5 to walk around and Resist vs. 15, 3d burning w/surge, metal is DR 1)
Further trap resources:
Steve Winter's 36 Trap Triggers
Steve Winter's 36 Trap Effects
C's Traps and Tricks Index