Honestly cool to see someone else having a go at it. The Dungeon keeper mechanic is so great and lends itself to lots of interesting ideas and possibilities.
did you ever work with tilemaps, and get into doing a "size/area" as dungeon keeper did, to work out room efficiency. what creatures share a bedroom.
I don't think i ever got as far as proper room construction unfortunately.
I think i used multiple tilemaps over the top of each other to show different graphics, then arrays to hold info on each tile coordinate, like if it was a floor, or wall, wall/rock type with each type having its own health remaining so it takes a different amount of time to dig, number of imps digging at a specific tile etc.
I coded my own floodfill, but theres this https://www.construct.net/en/make-games/addons/252/tilemap-flood-fill extension. Not sure what features it has.