When I became interested in chickens, I began making lists of what my coop/run NEEDED to be to keep them safe.
My run is completely enclosed w/ heavy duty, chain link fencing, it's buried 2 feet deep and placed in a two foot wide ditch and laid in an "L" shape then covered with cement and back filled with dirt. There's over 2000 screws, each seam is covered by 1x6 decking boards, bolted on and screws every 3",the wire is attached by huge fence staples. I have a 3' apron of 1/2" hardware cloth around the entire bottom... I have three locks on my gate,cement is poured under the gate to keep anything from digging under. We used 6x6 posts and 2x6 on the roof to attach the fencing to.
BUT I would never leave the door open to my coop at night.. I have chain link fencing and 1/4" heavy gauge hardware cloth on my windows, it's made out of 1" plywood and wooden siding, has a cement floor and a roof of metal over almost 1" plywood--everything is screwed in/together And I still worry about some determined predator getting into the coop......