It's a nine level dungeon—but the levels ascend up to the top. It's got features that are not in the engine; not in the base game. I designed/modeled/scripted ladders into the map for my room, and then we put them in other spots throughout the dungeon. Another fellow modded in a pair of blue and orange wands that work like the Portal gun [to a limited extent]. There are puzzles that have their own UI; rendered with the [developer LUA/script] graphics API. It has new spells, with custom spell effects. Lots of new asset models, and original monsters; both re-skins and built from scratch.
There is a fully functional player journal —similar to the one in Stone-Keep, but it's complete with drag & drop scroll storage, as well as supporting user notes. There is a quick-bar [custom UI], for readied potions.
The central hall extends all the way up through the nine floors—the player can peer up, or down the expanse; even toss things down from above. When designing it there were no tools; we had a fellow write stand-alone model/format converters to be able to import custom models and animations to the native formats, (he now works at Paradox).
It took us over a year to make it; each modder made and tested their own room, then about five of us bug tested the entire dungeon.
It has an overarching mission, but each of the 22 modders made a room that was up to 9x9 squares, and with up to three levels in height. None of the rooms are related; each is an adventure unto themselves. It's a —very— dense map. We had to erase and rewrite the level/rooms on the fly via script while inside each of the rooms, so that the frame rate stayed reasonable.
My room is: The Foundry
*Also... the design is such that not all of the rooms need solving, or even visiting, and those visited can [usually] be exited without restriction; no player is trapped inside, or unable to progress. If they are stumped, they can visit a different room for what they need.