Another thing you can do, after any other improvements you make, is sprinkle baking soda on the ground around the outside of the pen. Use a sieve to get a light, even coating.
In the morning, you will be able to see foot or pawprints if anything comes around.
A raccoon can open a door like that easily if it is not locked. Maybe that is what you are dealing with. I see you have a barrel bolt. Cats, dogs, and coons can open barrel bolts. A lock will definitely help. To keep the bottom of the door from being pulled out (if you can pull it out with the door bolted), you may want to put a lock farther down, too.
If you are dealing with coons, you may need to add another layer of wire that is stronger than the hardware cloth you have up. Some 14-gauge 2"x4" or something. Coons can tear hardware cloth.