We have a high predator load here that includes at least 2 coyotes; so I took that into account for the design. I buried a 2’ wide apron of 1/2” mesh welded hardware cloth all the way around the perimeter. The inside edge of the wire mesh is attached to the timbers that form the foundation. Inside the run was filled with a yard of tamped gravel with about 1 1/2 yards of coarse sand, watered and tamped. The back piece of buried wire mesh is most vulnerable since it is along the fence and out of regular view, so I buried it with soil, then covered that with large rocks.
As a result, Wyle E. Coyote couldn’t dig into the coop, at least not quickly. I used 19 gage welded wire for the walls, which is really strong stuff, and I attached it solidly with screws and grometted washers.
I have neighborhood cats, ‘possoms, and the coyotes; and no problems yet. I did catch one of the local squirrels inside the coop yesterday, checking things out. The hens were out in the yard and he just waltzed right in like he owned the place...