What makes Faerûnian dungeons different from dungeons in other worlds? History, mostly. No matter what its original or current function, dungeons in Faerûn will bear the stamp of its original builders and of the various groups that have occupied it over the centuries. Few dungeons in Faerûn stay empty for long.
No matter who built a dungeon, portals may connect it to other dungeons the same species or group has built, forming a network that past residents once used and that now provides player characters with a quick way to hop froin dungeon to dungeon. Likewise, many dungeons contain connections to the Underdark.
Adventurers: This category of dungeon builders includes mad wizards such as Halaster, the architect of the vast dungeon of Undermountain, successful adventurers looking for safe places to stash their wealth, and enterprising folk who build safe places to hide from their enemies when things get rough. Adventurers' dungeon features vary widely. Areas protected against scrying and astral travel are common, as are portals leading almost anywhere.
More bizarre features might include zoological or botanical collections (sometimes contained, sometimes running amok, extradimensional prisons holding fiends or elementals, labyrinths and traps built to guard special treasures, complexes of miniature chambers accessible only to visitors who are magically reduced in size, and vast laboratories containing deadly experiments still in progress after centuries.
Beholders: Beholders frequently carve out underground lairs using slave labor (courtesy of their charm abilities and their disintegrate powers. Beholder lairs usually feature vertical architecture that can be hard to negotiate without flying or levitating. They are often stuffed with magical oddities and other treasures.
Dwarves: The premier dungeon builders of Faerûn, dwarves build everything from simple mines to underground cities carved out of bedrock. Dwarf-built dungeons usually have fortified surface entrances in mountainous or hilly areas. Later residents who have the means to keep the defenses in good repair usually take advantage of them.
Even small dwarf-built dungeons feature living areas with safe water supplies and storage areas for food, trade goods, and weapons. Permanent settlements usually feature temples and catacombs, heavily protected by stonework traps, magical runes, and guardian constructs such as golems.
Dragons: Dragons sometimes excavate their own subterranean lairs, but more often they move into natural caverns or force weaker residents out of dungeons they have built. A dragon-occupied dungeon will have at least one big, lofty chamber or cavern that allows the dragon to fight on the wing. Unless the dragon can teleport, its lair also will feature an entrance that the dragon can fly or swim through.
If the dragon has allies or servants, they will be housed nearby in some area where the dragon can access, such as a side cavern or surface village.
Drow: Unlike dwarves, who tend to construct dungeons from the surface down, drow build up toward the surface from the Underdark. Drow dungeons serve as jumping-off points for raiding, trade, or colonization. They usually contain only spartan living quarters. Supplies (including water) are usually stolen from the surface or carried in from the Underdark. Drow dungeons often contain holding cells for prisoners and slaves.
Halflings and Gnomes: Halflings and gnomes burrow into hill sides to create private homes, mines, or small protected communities. Both favor forest settings over hills and mountains.
Humans and Other Surface Races: Many dungeons in Faerûn are simply the lower reaches of structures built on the surface. The subterranean levels of these buildings often remain long after the main structures have been destroyed. These dungeons can be exceedingly dangerous to explore. They may prove structurally weak and prone to collapse, especially when adventurers toss around fireball and lightning bolt spells. Many of these dungeons include old catacombs, which tend to be heavily protected with traps and magical wards but contain very little treasure.
Power Groups: The various organizations and secret cabals of Faerûn construct all manner of dungeons to serve as safe houses, training centers, fortresses, and way stations for members on the move. The more active dungeon builders include the Cult of the Dragon, who are constantly seeking out sites to house new dracoliches, and the Zhentarim, who favor secret fortresses and out-of-the-way spots that contain existing portals.