Complex sentence: The eastern side of the Basilica was built in a semicircular apse which Maxentius added two side chapels, on the west side, Maxentius built an atrium to the basilica and baptistery.
Split into simpler sentences: The eastern side of the Basilica was built in a semicircular apse to which Maxentius added two side chapels. On the west side, Maxentius built an atrium to the basilica and baptistery.