Complex sentence: Four piers support the weight of the dome at the central crossing; the piers of the four apses that project from each outer wall are also strong, to withstand the outward thrust of the half-domes upon them.
Split into simpler sentences: Four piers support the weight of the dome at the central crossing. The piers of the four apses that project from each outer wall are also strong, to withstand the outward thrust of the half-domes upon them.