My first priority when it comes to the chickens is predator protection. Although there are a lot of other things you need to consider to keep chickens happy and healthy, nothing can destroy a flock faster than an animal getting into the coop and killing them.
When I built my coop I was paranoid about coyotes and racoons so I tried to make the coop as impregnable as possible. I used 1/4" hardware cloth on the run and door and even buried it under the ground around the perimeter to keep varmints from digging their way in. So far it has worked and nothing has gotten in.
What I didn't plan for was the chickens literally undermining my efforts. All of their scratching in the run has moved enough dirt that there was a visible hole leading to the outside. It was still covered with hardware cloth and I piled some plywood over it but it wouldn't take much for an enterprising racoon to discover it and find it's way in.
Here you can see the section that the chickens had excavated. The whole area under the hardware cloth was an open cavity.
I used pieces of sandstone and concrete that I had dug up while building the lower stairs to fill in the gap.
I also used some of the excess dirt from the stairs to pack it all in tightly.
Here's the view from the inside. I'm hoping that the big pieces of stone will keep the chickens from digging their way out again.
Then I covered the whole business with more dirt. Supposedly most animals will try to dig right up at the edge where the plywood is and will be stymied by the wire mesh under the dirt. So far so good.