Complex sentence: Whereas the previous buildings had all featured a wooden roof, Bishop Guillaume's new nave, which today survives largely intact, incorporated stone vaults-a feature which required considerable thickening of the old walls and the addition of flying buttresses along the flanks of the nave.
Split into simpler sentences: Whereas the previous buildings had all featured a wooden roof, Bishop Guillaume's new nave, which today survives largely intact, incorporated stone vaults. This feature required considerable thickening of the old walls and the addition of flying buttresses along the flanks of the nave.