The finely arched front doors are often at the top of a sturdy staircase over the street-level cellar.
At the top of staircases, construction of doors was not possible due to narrow space.
mnli hypothesis: At the top of staircases, construction of doors was not possible due to narrow space. premise: The finely arched front doors are often at the top of a sturdy staircase over the street-level cellar.