Complex sentence: The present ground plan was established in the 13th and 14th centuries, from which time the church's external appearance dates, and from a major rebuilding in the later part of the 15th century, when the clerestory was added.
Split into simpler sentences: The present ground plan of the church was established in the 13th and 14th centuries. Its external appearance dates from this time, and from a major rebuilding in the later part of the 15th century, when the clerestory was added.