I just discovered this post lurking in my site, unpublished. It's a very long time since I wrote Trappery, and any train of thought I had going is long since dissolved. So this is a pretty short post, but it looks coherent so I thought I'd stick it out there rather than deleting all that work.
The Goblin Caves
In contrast to the last example, this one is far from reality. A band of utterly stereotypical goblins has holed up in a cave network. They are cunning, malevolent, nasty, brutish, and short. The goblin chiefs, led by Ugluk, want to protect their slightly-less-decrepit belongings from the light-fingered masses, as well as from any thieves that might come sneaking into their territory.
The main objective is to protect the chiefs’ belongings, by discouraging thieving attempts or by stopping any thieves. The targest are goblins, any other humanoids that might wander in, and possibly the odd stray animal. The only people expected to enter the chiefs’ huts are the chiefs, and the odd favoured goblin who might be escorted in there by the chiefs. There’s a fairly high risk of theft, both because goblins are treacherous thieving swines, and because this is a world full of kobold bandits, orc bullies and sociopathic Neutral Good humanoids who like nothing better than relieving goblins of their ill-gotten gains.
On top of that, goblin life is hard and violent, with pain and death constant companions – they’re far towards the bottom of the XP chain, and so they don’t place too much value on life, either others’ or their own. Certainly, goblin chiefs place far higher value on their possessions (and privilege) than on the lives of their subordinates, and they’re willing to face the risk of injury to keep those possessions secure; there’s also no legal issues about killing off thieves. This isn’t a military force as such, but we can certainly consider it an official group with access to whatever the goblins have. They’ve got a limited budget and fairly limited technology; they also can’t trust their own kind, so they’re left relying entirely on mechanisms unless they go for guard animals. A trap placed within the caves is relatively simple to maintain or inspect if it triggers.
Ugluk will rely heavily on deterrence to keep the other goblins at bay. The mere threat of traps might work, but goblins are pretty stupid, and of course there are intruders to consider. That means some kind of obvious deterrent has significant advantages. They want to make sure would-be thieves know they’re asking for a mutilating. Being cunning and treacherous, though, they’ll almost certainly add in a non-obvious trap just to be sure that anyone who does try their luck runs out of it. And maybe a third trap to catch anyone who guessed there would be a second.
An obvious trap needs a difficult bypass method, otherwise what’s the point? That’s either a secret, or a token or ID that’s hard to obtain. Something like a falling spike trap along a corridor could rely on a sequence of trigger stones, which only the chiefs know. Alternatively, they could inscribe runes around their hut, which strike any intruders with lightning unless they’re carrying one of the chiefs’ enchanted torcs. An ordinary goblin band probably won’t have the advanced magic that allows for individual identification. The chiefs have to put up with some inconvenience, but they’re willing enough to do that for the sake of protecting their treasures. If we don't want to consider magic at present (and there are probably some differences we should look at), there are options like a big spiky trap that needs a special key to pass through.
A less obvious trap might be a concealed pit, a false lock that triggers a dart to the face, or a warding spell on the chiefs’ treasure chest that releases a cloud of toxic gas unless the magic word is spoken first. Unless an intruder is looking for something like that, they’ll be caught unawares and painfully killed.
When worrying about outsiders, goblins have some obvious advantages. Most intruders are larger and heavier than a goblin, which means the goblins can design traps that a goblin can wander past in relative safety, but which will be easily triggered by a human, dwarf or orc. The perimeter or vital locations can be guarded by trapdoors, or by mantraps that a goblin's weight won't trigger. Trigger wires can be set above goblin head height. Perhaps a narrow doorway has pressure sensors around it; a goblin passes through readily, but a human crouching to squeeze through will tend to brush them.
You could also design a cunningly-balanced lift mechanism. A single goblin can use the counterweight to rise to the chief's elevated chambers, but a heavier humanoid outbalances it and sinks into a prison chamber. Of course, two goblins would have the same problem, but a sensible goblin chief wouldn't trust two goblins at a time anyway!
Oh, hey, here's a fun idea. And it's very appropriate for goblins, kobolds, and the other sneaky devious races.
From an adventurer's point of view, here's how the trap looks:
You creep into the goblin caves, towards the inner chambers. Scouting ahead, the rogue spots something in the dim, rubbish-strewn tunnel: a pit trap, amateurishly concealed beneath some debris. It's about 10' wide and quite deep.
A good [insert appropriate skill] roll reveals more, though. The rogue realises that the faint footprints in the tunnel all go straight up to it - and appear to continue on the other side. Peering closely at the pit, she discovers that it's actually a shallow depression, painted to look like a pit. This is a surprisingly skillful bit of reverse camouflage. To either side, there are deep, well-hidden pits covered by dusty cloth. It looks like the goblins use this simple bluff to lure intruders into their pit traps.
If any Medium or larger party member tries to walk across the false pit, they fall into a 20' pit. Several cords at the bottom lead out through small tubes (likely made of bamboo or clay) and attach to large bells some distance down the corridor. The falling character will inevitably jerk the cords and sound the alarm, summoning the nearest sentries.
The trap is a 20' pit, covered with a lid of clay, dried mud, wicker, bones or whatever else seems appropriate, which is then cunningly coated with a couple of inches mud and dust to seem like part of the tunnel floor. It's built so that the lid is about 1' below the tunnel around it. The earthy lid is then carefully painted with a picture of a deep, spike-lined pit, and badly disguised with debris. Goblins using the tunnel walk over the pit one at a time, as their weight isn't enough to break through the lid. If they need to pass this way with anything heavy (rather than take another route) they carefully lay planks across it.
Adventurers who don't notice the pit will fall into it. Adventurers who notice the pit will walk around it and fall into the well-disguised pits next to it. Adventurers who notice the well-disguised pits and footprints will realise what the goblins do and copy them, only to fall through the pit and raise the alarm.
A better skill roll will reveal that the depression surface doesn't quite look like the surrounding ground, and that maybe the adventurer should test it out. Knocking on the lid produces a hollow sound. Treading on it carefully hints that it's not quite solid enough.
But maybe that's just not devious enough for your campaign? Fine. Let's go fifty feet down the corridor.
Oh hey, it's one of those false pit traps again! Once again, it's surrounded by other well-hidden pits, this time with spikes. This one is wider, though, about 15' across. The lid is a little cruder, too. And looking around, the adventurers will discover some long planks carefully tucked onto a ledge. These are covered in goblin footprints, and well-used. Obviously this is how the goblins get across the pit.
If the adventurers fail to notice the pit and walk straight across it, they fall into the pit. Adventurers who notice it and use the planks to get across discover that they have been cunningly sawn through, and anyone larger than a goblin using them will cause them to break halfway across the pit, plunging them into it.
Adventurers who somehow make it across the pit can follow the goblin footprints to a pond of stagnant water, with a small island at the far end. On the island is a wooden idol holding a glittering gem. The pond contains various nasty aquatic creatures, and apart from occasional trips to keep the footprints fresh, the goblins never go there. The totem is a decoy enchanted with magic mouth to pronounce a terrible warning, and the gem is a piece of glass (if you're feeling particularly cruel, it's inscribed with explosive runes).
Careful examination of the spike-bottomed pits reveals that most of spikes are another cunning bit of artwork. There is a tunnel in the wall of the pit, not easily visible from above; the goblins use not-very-sharp spikes in the wall to clamber up and down.