The inspiration was a desire to make the stonework I was wrapping around my outlines as part of making it prettier matter more by putting texture and intrusive shapes into the rooms and corridors, so there would be built in tactical nooks and crannies for thieves and stealthy monsters to hide in on otherwise plain walls, etc.
Pencil in guidelines against a straight edge. On the third one I used a rolling ruler that could double as a compass for curves. Break up the simple grid by doing two or three of grids at angles, a stray extra guideline at an off angle, a spray of radii out from a point, or some arcs.
Mostly running along grid lines draw chains of stones. Vary size and shape both in runs and patterns to maintain visual interest. Wiggle off the lines, and occasionally meander a diagonal or curve for more interest. I used a 0.3 Staedtler Pigment Liner for these. Have started also experimenting with the 0.1 for stonework.
Do some minimal stippling to show what is solid. I used the Staedtler 0.1 for this.
Erase the guidelines with a good white artist's eraser.
These were done on a big pad of Canson 1557 Drawing paper.
It is a surprisingly easy style to do and goes pretty quickly since the stippling is so little it kind of makes up for doing the chains of stones instead of simple lines. It it both pretty forgiving of minor goofs and the slower pace of making a line as a chain of stones puts me in a contemplative mood for where the lines should go, and there is an inherent roughness to the resulting stonework so I don't find myself regretting a misplaced line as often as with more conventional draw the outline and then stone + stipple or hatch around it.
All three will get further detailing when I stock them. For now they are missing doors, floor textures, statues, objects, etc. I like them as is, since they have such a ruin look to them. Kind of an archaeological sketch vibe, Put in a few ways to go from one level to the another and they'll make a nice midsized dungeon.
I'll probably stock them for D&D 5E. Also thinking about making some 2.5D terrain pieces that would fit together to depict parts of these for miniatures.