Complex sentence: The roof above the three-aisled hall had three parallel vaults with an oculus in each, which were supported by arches that stretched from pillar to pillar along the aisles, and went over the aisles from the pillars to the walls.
Split into simpler sentences: The roof above the three-aisled hall had three parallel vaults with an oculus in each. The vaults were supported by arches that stretched from pillar to pillar along the aisles, and went over the aisles from the pillars to the walls.