The trap is a D&D icon. Classic dungeons such as Tomb of Horrors and The Hidden Shrine of Tamoachan are famous for their mechanical ambushes. Traps are so central to the concept of dungeons that an entire class—the thief—was developed to deal with them (along with locked doors).

In real life, of course, archaeologists have never had to deal with this abundance of traps in ancient tombs and ruins. No trap of any kind has been found in an excavation: no scything blades, no darts with pressure-plate triggers, not even poisoned needles in treasure chests. Sliding blocks have been used to seal passages but never to squash intruders. Deep pits were dug in the entrance corridors of some tombs, but they were meant only as obstacles. The pits weren’t covered, so only the most irresponsible of thieves risked falling in, and none of them left behind skeletons with broken legs. Tomb architects went to great lengths to keep people out, but no thought was given to killing them once they got in.

GMs need to put some thought into it. Before putting a death trap in that hallway, you need to ask exactly what it’s for. Designing a clever, challenging, logical, dangerous, and yet enjoyable trap is one of the toughest tests facing a GM.

This simple diagram looks at a trap’s purpose from two angles: whether characters are meant to spot the trap, and whether they’re meant to trigger it. When placing a trap in an adventure, the GM should know the answers to those two questions.

Obstacle traps are meant to be found but are devilishly hard to disarm or avoid. The characters will either spend time and resources dealing with the trap, search out a detour to go around it, or trigger it and take the consequences. In all three cases, the trap achieves its purpose. Obstacle traps aren’t really traps at all; they’re problems or puzzles to be solved, presented in the guise of a trap.

Death traps are meant to be found and avoided. No GM really wants to kill characters with a trap they didn’t know was there. A fatal gotcha like that serves no purpose. A death trap is most effective when characters discover it, understand that it’s 98% lethal, and figure out a way to avoid it. Dodging that bullet can be one of the game’s biggest vicarious thrills.

A trap that’s meant to be overlooked and triggered is a story trap. Triggering it advances the plot in some important regard, possibly in a way that only a trap can. It might be how the characters fall into the clutches of slavers, how one of them is affected by a curse, or how they’re dumped into a portion of the dungeon they’d rather avoid. Story traps don’t kill and seldom cause serious injury. What they do is change the situation in unexpected ways.

Characters aren’t expected to find or trigger traps in the fourth category—back traps. It might seem that these serve no purpose, but that’s not necessarily true. A back trap is perfect for an area that characters are expected to pass through multiple times—say, on their way into the dungeon and again on their way out. Players assume that an area is safe if no traps are spotted and none are set off when characters move through. With a back trap in place, they’re in for a shock on the return trip.

Of course, the back trap doesn’t need to go off then, either. When characters meet a back trap on the rebound, it can function as an obstacle, a story trap, or a death trap. A death trap is doubly effective in this situation, because players won’t miss the fact that they walked blithely through it once.

This system is just one of many ways for a GM to look beyond a trap’s trigger and effect to get at its true role in the story. Which one works best for you?