Symmetry?

Today I was in Leiden.
I took a picture of this detail of the 'Hooglandse' (highland) church, because I love lines and symmetry. When you pay some more attention, you will see two doors next to the white structures. It is a pity that they break the symmetry though .
I asked myself 2 questions: why are these doors there? And: how can they be locked from the outside?