Complex sentence: Located under the floor, the heating system (hypocaust) allowed the air, warmed by wood fires in adjacent vaults, to circulate between pillars of terracotta disks heating the marble floor; terracotta pipes in the walls allowed the air to rise up to the ceiling.
Split into simpler sentences: Located under the floor, the heating system (hypocaust) allowed air that was warmed by wood fires in adjacent vaults to circulate between pillars of terra-cotta disks heating the marble floor. The terra-cotta pipes in the walls allowed the air to flow and rise up to the ceiling.