You know, those labels: Suitable for 2-6 adventurers of 3-5 level. Sure, there's always some wiggle room, always hyphens because party power is hard to determine if you don't know which classes are present and how well the players rolled their hit points. But it still feels a little tidy to me. It gives me the impression as a player that I will encounter nothing I can't handle. It constrains the Mythic Underworld.
My challenge to you all, give me an adventure locale suitable for all levels.
Some thoughts on this.
- Hirelings can serve as a powerful DM tool here. They can be a buffer that reduces party size as a factor. They can also help bulk up the power of a lower level party through sheer numbers.
- Part of the problem here is thinking of all encounters as combat encounters. Who says the dragon even cares about your first level schmucks? Maybe the Sphynx likes talking to visitors and will only eat you if you annoy her, whatever your level.
- There would definitely need to be decision signposts for players, so they didn't stumble into packs of beasts that do only want to eat them.
- Geography is probably the key. Think of the Caves of Chaos but also how other topological features of a place might partition different monster communities- elevation, water sources.
- Verisimilitude could work to our advantage here too. Maybe the lower level creatures work for the higher level creatures. Maybe the powerful planar creatures are observing the ecology of the place, and that's why they don't blast it all to kingdom come.
- Puzzles are difficult for players, not characters, and thus should be level neutral.
- Environmental hazards- rock slides, lava, geysers, mud pits- could make places equally deadly regardless of level.