The walls were built in the short time of only five years, though Aurelian himself died before the completion of the project. Progress was accelerated, and money saved, by incorporating existing buildings into the structure. These included the Amphitheatrum Castrense, the Castra Praetoria, the Pyramid of Cestius, and even a section of the Aqua Claudia aqueduct near the Porta Maggiore. As much as a sixth of the walls is estimated to have been composed of pre-existing structures. An area behind the walls was cleared and sentry passages were built to enable it to be reinforced quickly in an emergency.