I love ruins. I know; they can be cliché, but I love ‘em. It comes from actually having read Jungle Book and Tarzan and the City of Opar. Whether it’s a ruined temple with a vault of incredible wealth in the basement or a vast deserted city filled with minor clues as to what really happened to the mighty inhabitants. I guess it’s because I think it’s a lot more plausible that people would wander around in a massive ruined city than they would find an underground dungeon stocked with monsters. (Sorry if I’m being a buzz kill.)
How much do I like them? An entire continent on Fletnern is now a ruin filled archipelago. I wasn’t satisfied with a ruined city, I needed a ruined continent. The biggest adventure setting I ever made? The ruined city of Ballogfar - Capital of the Goblin Empire. Ballogfar was actually a fairly well constructed idea. I wanted a big ruined city - but what to fill it with? Avoiding the whole Shangri-La idea of a lost city filled with people, I wanted a true ruin. So the only thing that survives for 1,000 years is undead. Of course, I could have gone for the whole animals moved into it, but that wasn’t the spooky ruin vibe I wanted. So what happened? Well, the ogres, orcs and goblins lived together in a caste system, but then the orcs and goblins rebelled against the ogres. That seems logical. So the ogres pressed on with some goblin slaves and a force of crafted undead slaves. That too seemed reasonable. But having an army of zombies walking around eventually bred a plague within the city, wiping out most of the ogre population and most anyone who wandered into the city. This justifies the massive size of the city (it once housed tens of thousands of goblins, orcs and ogres) as well as the reasons it hadn’t been found (those who found it were dead of the plague before they could return to civilization).
Of course, you don’t need that massive size - a ruined temple works great! Religious folks built a temple and a small village around it. The temple is probably stone or brick, while the homes were wood. When the neighboring cult wipes out this cult, their temple is seen as taboo. One hundred years later, the jungle has swallowed the village and nothing remains, except the temple, now filled with snakes and that one fabulous treasure that served as a embodiment of their god. (Don’t tell the adventurers that the jewel is cursed - they’ll figure that out later!)