In the entry for this building in Wikipedia, it shows the surviving part of the old church (which was bombed by the Allies during World War II) with a large bell-shaped hole in one side. Was there a window there? I know the round rose window is also gone…
I think what you’re seeing is a vault arch–maybe the bottom of that crossing tower. The rest of the building that was attached to that piece of ceiling is gone, so you get a cutaway view of the vaulting. Make sense?