Guns of the Lost World

This painting by Sanjulian packs a lot into one image! It suggests (to me, at least) a cross-genre campaign setting: a lost world like Ka-Zar's Savage Land, Turok's Lost Valley, or Warlord's Skartaris, where dinosaurs still roam and lots of Edgar Rice Burroughs-esque lost cultures are to be found.

Maybe this place is found in a hidden valley in the Sierra Madres, or maybe its an underground world accessible from the Superstition Mountains (where Apaches say (supposedly) that there is an entrance to the underworld) or the Grand Canyon (we've already had reports of strange artifacts). Where ever it is, here's what I think the picture suggests you'll find:

Dinosaurs: From all shorts of eras, cheek and jowl with prehistoric mammals and humans.

Conquistadors: Several groups of Spanish explorers found their way into the lost world. Some are undead thralls, toiling in the castle of an alchemist. Others are immortals zealously guarding a fountain of youth that just might be an alien artifact.

Primitive tribes: Descendants of Native Americans, a Lost Tribe of Israel, Phoenicians, Vikings, and maybe even ancient Romans. Most have reverted to paleolithic levels and are at the mercy of the monsters in their world. A few do interesting things like tame pteranodons for mounts or sacrifice captives in the name of some cargo cult.

Giants: Remains of giants used to be found in tombs all over the U.S. These primitives (Nephilim descendants, probably) are mostly more belligerent that the regular sized primitives, and the one in the picture at least has a sword. They may often be solitary and have unusual powers like the ability to call lightning or command a pack of wolves (or hyenadons).

Humanoids: Some of the human tribes degenerated so much they because something other than human. Maybe its simple degeneration, or maybe is exposure to weird radiations from a long-buried alien spaceship, or maybe it's worshiping dark gods? Or maybe all three? Whichever, they're almost universally hostile and creepy. Some of them are probably Dero.