Start with a simple map layout. One vertical corridor and two crossing horizontal halls. Put a few rooms into the spaces between the passageways. Then greeble the hell out of it.
This portion of the Rahnlaine Dungeon was constructed during the Bistich Hegemony, when crenelated construction was in favor over long unbroken walls (since it was much harder to write out opposing Dravellar slogans on irregular surfaces). Though of no practical benefit in underground warrens, the style was adopted even for the extensive works beneath Castle Marthrikt and the keep of the city.
This space is ideal for encounters with small creatures that can readily hide in the shadows and the many niches and corners.
These halls extend many hundreds of feet in all directions, with many connections to the basements of buildings and to the fortifications around Castle Marthrikt and the city.
As is the case with most of my maps, feel free to use this for any non-commercial purpose (with attribution). You can also contact me if you’d like to use any of my works for a commercial project.