It depends on how you're making the maps/what software you are using. A simple way in photoshop/gimp/etc. is to make a selection of the area you want your walls to be in and fill it with a texture, like stone or wood. See here for some ideas (all of it may not be relevant, but there are a lot of little tips and tricks in this tutorial): https://www.cartographersguild.com/s...ad.php?t=27077