Complex sentence: During the years 1767-1768, the double arch was opened between the western tower and the middle one, which (ST: No queda clar qui és l’antecedent) gives access to side façade of the church through a stairway.
Split into simpler sentences: During the years 1767-1768, the double arch was opened between the western tower and the middle one. This arch gives access to side façade of the church through a stairway.