You may well have heard of the RPG cartographer Dyson Logos as you'll find their art used lovingly by the gaming community.
This article covers not only designing/refurbishing a stronghold but also how to use the rules to make it happen.
Basic World Building uses the tools from 1981’s Basic Dungeons & Dragons and Expert D&D and re-imagined in Old School Essentials to build a starting campaign setting.
There's a page on Dyson Logos that you should know about.
Summon up a dungeon (or appropriate map) in seconds.