we have chickens and a very secure back garden. (5-6 foot high solid walls)
For the first year or so we just let them free range. The main disadvantage is that they really do cover the ground in poo, so using the garden became significantly less pleasant.
They also loved to eat the ripe growing tip on all the plants, so anything that was too small disappeared.
I have seen foxes in our garden overnight/at dawn and dusk. I honestly don't know how they get in (see what I said about a wall)
Eventually we got 2 runs. One is a self contained house/coop and run, all attached. It wasn't huge, so dh took the back off and added a couple of metres, so now it is much bigger. This is effectively a wire box, with very small tough wire fencing, and a wire floor. it is totally fox proof. When we are going to be out during the day, or when we are on holiday, this is their home. we open the door of the nest box/coop and they can potter round in here all day long, and are 100% safe. they can also put themselves to bed, and the coop door doesn't need to be sut if it isn't cold, so in August, my friend can come in when it suits her, and replenish the food and water, and the chickens are fine.
The second run is basiclaly 1/4 of the garden fenced off. So when we are at home, we open the door of the fox proof run and they potter ournd that corner of the garden. there is a flower bed, tree, gravel, wood pile etc, and they can scratch in the dirt. The disadvantage is that someone has to be there at dusk to shut them up for the night.