I believe that, first foremost, you have to create a throne room and build a floor in the middle of it and build an oubliette under it. There RS gold is also the need to construct a trap door in there. After that, there are two options for getting into it An enormous cave entrance you construct in a garden/fancy one. A flight of steps in the middle of a room.
After you've completed one of those two rooms, you can choose to build another room below in the same way, but there should only be one option for that. After you've built that room(s) it is possible to construct in normal mode, creating treasure rooms, dungeon hallways, junctions. You can also build any room from above you would like to have down below too.
Also, if you've not done it, this Love Story quest unlocks a unique dungeon-like room which comes with a multitude different dungeon guards are available to add which include scarabites, iron dragons, black demons and more., and cheap OSRS GP most of these new creatures are cheaper than the original trolls, hellhounds and steel dragons.. If I've missed anything, please feel free to ask again.