Thanks for the compliments.
Sara, there is a door for the open window pane. Otherwise the chickens would fly out whenever they wanted (They are only allowed to be out when I am home). So, that should keep the owls out.
Lee, the barn is not post and beam, I think it's balloon frame. My house is post & beam, though. (My house is older - c.1855.)
It seems to be working out pretty well. The chickens seem to like the elevation. When I first put them up there, I didn't have the 8-pane window in place yet - I just had a removable screen across the whole window. I would put the chicken ladder up, and take out the screen. But they would ignore the ladder. They would just jump out the window -- and crash! I think that is why they are called Barred Rocks. They fly -- a little like a rock. Thud.
Now that the ladder is directly in front of the "door" they are better at using it.
The platform they hop up to is about 5' off the ground, and this seems doable for them, both up and down.