Mirkwood

D&D setting idea; an immense forest. It's basically Necromunda through the filter of Robin Hood. Most communities - most safe communities - exist halfway up the immense towering trees, connected by roads across the limbs. Limbs so thick the lightest breeds of giants, the ogres and ettins, can potentially travel. Limited arming is possible, foraging and hunting possible, defense is relatively easy. Outside of those communities, higher up are little halfling and elf dwellings, even harder to reach and as you go vertically lower and lower to the forest floor things get progressively wilder and more dangerous. To one direction the woods slope into bog and lake, in another a tangled labyrinth of thorned trees from houseplant to oak sized, none of which are in any way safe places to be. This place used to be an elf-woods until several centuries ago the elves simply disappeared, leaving behind haunted ruins at all levels, and a few outlier families whose descendants might hope to reclaim the place for elvenkind. An opposing power wiped them out in one night of terror and rage, and there are still extraplanar creatures around from that invasion. As well as the Heart of Winter, an area where summer never comes, the frozen remains of the last elf citadel to go down.

The disparate races that inhabit the forest are almost totally devoid of clerics, relying on druids. Being neutral druids sort of ease the tension between groups which might otherwise bring bloody war from floor to canopy. Clerics are bad news. There is one foe which nobody, human or bullywug, halfling or gnoll, wants to see in their forest and that is outsiders. Powerful feudal lords mass outside the place, their forces bolstered by clerics of a pseudo-Christian religion, and they'd have taken it long ago if they didn't like having the wilderness blocking travel - or if taking the place wouldn't be a bloodbath.