After you've completed one of RS gold 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 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.
Lardar pretty much cheap OSRS gold summarised the most effective methods, however in order to create a plan , we'll have to know the level at which you're at.