The wall drawing tools in DD3 are usually lines of varying thicknesses, I think, which makes it harder to disguise their pristine nature without some extra work - adding rubble symbols, for instance, and maybe trying some additional Sheet Effects on them (as well as Monsen's Color Key suggestion), to give a more "worn" appearance.

Loopysue's suggestion is a good one too - using symbols to make the walls in the first place is a definite alternative. There's the Dungeon Walls Annual issue from 2018 which uses exactly this technique, for example.

Or you could try drawing the walls using polygons instead of lines. That way, you can give any breaks a more ragged appearance as you're drawing them, and even vary the thickness of the wall itself. That will probably need some symbols adding to improve the effect though. Ruins just aren't designed for neat-edged drawing tools!

There is an Annual issue from 2014, the Black & White Battlemaps one, which has a few ruined building symbols in it as well, though they're intended for larger-area overview maps than the usual dungeon maps - plus, black & white only, of course.

I've probably forgotten others too ?