Complex sentence: Evidence of Narbonne's sudden and dramatic change of fortunes is quite stark when one sees at the rear of the cathedral the enormously ambitious building programme frozen in time, for the cathedral--still one of the tallest in France--was never finished.
Split into simpler sentences: Evidence of Narbonne's sudden and dramatic change of fortunes is quite stark when one sees at the rear of the cathedral the enormously ambitious building programme frozen in time. For the cathedral--still one of the tallest in France--was never finished.