How do you guys handle enemies setting traps for the party? I'm designing this encounter where goblins have set traps in advance but want to know what's the DC for the party to notice the hidden trap (vs. Perception). Should I just assign a DC (per the DMG) or should I roll for the goblins trapmaking skill? What would that skill be? An Intelligence check?
For determining Dc i use the DMG guidelines unless i have other reasons to do so.
basing it on the enemy is great.
My general rule is:
DC10 easy for cases where the adversary is untrained (not proficient) ANDunexceptional (no strong ability)
DC15 Moderate for adversaries which are EITHER trained (proficient) OR Exceptional (strong ability)
DC20 Hard for adversaries which are BOTH trained (proficient) AND Exceptional (strong ability)
I raise or lower that by one level (even to 25 or 5) for exceptional circumstances that would provide advantage or disadvantage to the adversaries - mostly resources (long or short time to do the job, lots of workers or few, limited/plenty parts and goods that would help or be needed, weak/strong discipline and resource for upkeep once they get placed) etc etc etc.
Note that "trap" is a rather nebulous term and it really depends on the circumstances, setting and the objectives of the adversary.
A miscellaneous step-into-damage in the wild makes little sense in the basic high fantasy high magic world. it really does little as a rule. At best it slows the party down which is great **if*8 you have a chase scene going on but otherwise, who cares.
traps should be set to give you advantages you can exploit.
One of the useful traps is a simple alarm or sentry trap - one which will let you know by some means that their is an enemy and where they are and as such allow you to prepare and allocate resources. this *CAN* and likely should be combined with the others. Drums, noisemakers, fires that can be spotted all can serve this.
most of the other types are much less useful unless combined with attacks and tactics. Dividing the enemies, slowing the enemies, even some damaging is good when you can also be attacking and raining down damage. Something like a flood wash or avalanche/collapsing platform/bridge that not only carries the chance of damage but also (and maybe more important) splitting the party is wonderful - especially when there are attackers ready to hit either or both groups while split. (It may actually be better to intentionally fail the STR/DEX save to "hang on" rather than see your party divided into two groups fighting two sets of foes.
Spotting can be a passive perception or active perception or maybe even an investigation depending the PC choices. But sometimes for enemy triggered traps there may be little or no chance to spot it or at least very difficult one. There may instead be an active counter.
etc etc.
