Its a roof hatch.
Like you might find on a submarine.
It can be locked from the inside so having a handle on the outside isn't a problem.
In Company of Heroes, the older version, you could upgrade base buildings to have machine gunners pop up thru hatches in the roof.
It was that or a satellite dish, but there were no satellites during WWII, that we know of.
I just wish they made slope/corner pieces at that angle, I couldn't find any.
The red bricks in the back wall are air vents and have those ridges on them.