Complex sentence: That on the north side was built so it could bear a tower, but this addition was never made: instead, a belfry was added, "astonishingly situated astride the ridge" of its roof according to Nikolaus Pevsner.
Split into simpler sentences: That on the north side was built so it could bear a tower, but this addition was never made. Instead a belfry was added, "astonishingly situated astride the ridge" of its roof according to Nikolaus Pevsner.