We actually buried about 2 feet of extra chicken wire we had. My son dug a trench all the way around the edge of the coop area and we stapled the edge to the bottom of the fencing and coop.
We never had a digger get in.. but one time we forgot to lock the main coop door/gate outside and a neighbor's dog got in.. terrorized the poor chickens and ended up killing about 3 dozen birds.
Ouch, tough break!
Good idea with the wire. Im working on a better door method
We never did figure out the best door method. We just kept adding new locks and bolts at the top of the door.