Intermission: Traps and How to Use Them

Last session, the party narrowly avoided becoming the victim of a crude goblin trap - a big stick that swings into your face when you open a door. Fans of Skyrim will be very familiar with this experience. Traps are vital to the Dungeon Fantasy experience: they encourage careful preparation and exploration, are fun to work around and integrate into the party's own plans, and perhaps most importantly, they give the thief something to do! Thieves get a little shafted without traps in the mix, as swashbucklers, martial artists and scouts are often just as good at sneaking about, and the wizard can like as not just turn the fighter invisible. 

So, we need traps - but when are they appropriate? Which ones are suitable for the situation? What if I really, really need a trap, right now, because the party is going a way I didn't expect them to? For answers to these questions, read on. 

Categories of Traps

I divide traps into four main categories, depending on the ease of setting them up: hasty, simple, moderate, and complex. 

Hasty traps take seconds to minutes to deploy. They're for discouraging pursuit and controlling the field in combat, rather than actively killing people - although they certainly make it easier. They often use the enemy's own momentum against them. Good examples include caltrops, glue, slippery oil, alchemist's fire, and tripwires. Tricksy enemies (and adventurers) are fond of using these on the retreat to buy time, as they're often trivial to avoid once you see them, but the Perception penalty for haste makes them functional. Intelligent enemies can use these anywhere. 

Simple traps take minutes to hours to deploy. Traditionally used by combatants that are cunning but poor, such as goblins, bandits, partisans and - of course - party members. These include most of the traps found in DF16: bow traps, leg-holds, spiked branches, snares, falling nets, swinging logs, and small pits and deadfalls. They're almost always single-use affairs, depending on gravity or spring tension to deal damage or alert the trapper. Usually best used in natural caves or the woods, as these tend to have lots of nooks and crannies to hide things in, as well as soft ground to dig and set stakes into. These can also include classic door and chest traps - poisonous dust on the seam, or envenomed needles hidden in lock mechanisms.

Moderate traps are more architecture than trap, and only really make sense in deliberately constructed dungeons, and only then in places that won't see a lot of foot traffic (or else people would be setting them off as they come and go). These are usually powered by spring tension as well, making them one-and-done until they're reset by the unfortunate dungeon keeper's assistant, but often have dedicated controls and mechanisms to hold it for a long time. Examples include spears and crossbows that shoot through holes in the wall, trapdoors, swiping blades and falling ceiling tiles - pretty much anything that runs off of a pressure plate to activate falls into this category.

This can also include monster traps, which I'm putting here because they would require deliberate herding or a lot of time to reset. Falling slimes, trapdoor spiders, or my favourite, the massive pistol shrimp, are all good options here, although even things like swarms of murder bees in holes you put your arm into would count. 

Complex traps are your real Indiana Jones lethal affairs, generally reserved for ancient tombs, evil temples, and anywhere the builders really didn't want or expect anyone to be in. These are usually powered by magic (or mechanisms so complex and inaccessible that they might as well be magic), and include such wholesome wheezes as giant rolling boulders, trash-compacting walls and ceilings, and perpetually spinning blades whizzing back and forth along walls - although the sky's the limit with these devices. They're often less traps than set pieces, requiring the party to deactivate the mechanism or avoid them through cunning and skill. 

How to Use Traps

A trap's only as good as the trapper, and that goes as much for the GM setting them as the character. If you're anything like me, you want a trap that's fast but fair. The full write-up given in GURPS supplements is good, but I find it a little rigid - it's hard to tell how to adjust the penalties and damage in a way that feels right. 

1. First, pick a trap that fits the setting - magical laser traps aren't often thematically appropriate to the gnoll cave, at least not without good reason. Simpler traps are generally less dangerous, but can be fiendish in ways that don't deal damage directly. Slippery oil on the path to a sheer drop can be just as deadly as a big swinging scythe blade, and less obvious to boot.
2. Pick a detection penalty. This can be set by GM fiat or by a quick contest between trapper and victim, but a quick-and-dirty rule for this: imagine the IQ of the trapper, assume he has Traps and Camouflage at IQ level, and that he rolls a perfectly average 10 in the quick contest. That makes the penalty the trapper's IQ-10. Add on an extra +2 to represent extra time, if you feel that's appropriate - but note that extra time is scaled to the length of the original task. It's reasonable for the bandit to take half an hour, rather than 10 minutes, to set up a snare trap; less reasonable for the ancient architect to delay the temple's construction by 40 years because he wanted to get the rolling boulder trap just right. 
   
   For example, an IQ 11 goblin leader sets pitfalls that are detected at -1; an IQ 15 dungeon architect sets a hatchet launcher that gets detected at -5. If this seems low, remember to include darkness penalties, Per penalties for looking down (or up), curling mist, sheets of rain and foliage if you're outdoors... 
   
   As an aside, the DFRPG Traps book appears to base the detection penalty on the form of the trap, rather than the ability of the trapper - a -5 for a pressure plate, -9 for tiny holes in the wall, etc. This seems more reasonable for thieves with Traps-20 than for the thief fresh out of the box with Per-based Traps-15. I find it's more palatable to have many different reasons why the thief could fail to see the trap than a simple "it's small" - especially if they've run into something like it before. 
3. Pick a disarm penalty. Just use the same one as for the detection penalty - most thieves will find the trap easier to disarm than to spot (thanks to higher DX than IQ), but that's fine. P(B|A) and all that. Obviously, some traps aren't appropriate to disarm - use your judgement, but give the players an opportunity to come up with a plan even if they can't succeed. 
4. Pick a save. A common problem with traps is that you tend to have two kinds of people falling into them - thieves with no armour, and knights with all of the armour. The save is one of the big discriminators, as thieves are good at dodging, have high Per scores, and often have Danger Sense - so make the Dodge dependent on a Hearing roll (remember that pesky -4 for full-face protection), add a penalty, and make the attack come from somewhere DB won't protect from. 
5. Pick an effect. This might be save-or-get-stabbed, but recall who's going to be hit by it. A trap that launches a 1d+2 imp spear will be laughed at by a plate-armoured knight; a trap that swings a sledgehammer at 3d+5 cr will absolutely ruin a thief. To lessen the disparity, try traps aimed at the face; really pointy spears with an armour divisor; toxic gas or acid sprays; or effects that don't deal damage directly, like oil, glue, or paralytic slime. Also, remember that traps can be tailored to who's likely to meet them - a trap placed in the air ducts where a knight won't fit doesn't need to pierce plate. Finally, note that it's trivial to add a secondary mechanism that rings a big gong when the trap goes off, forcing a wandering monster roll. Irresponsible not to, really. 

Hopefully someone out there finds this helpful - I certainly would have if I was planning a dungeon not so long ago. Happy trapping!