Watch Movies2Watch. Despite their personal short comings, many of the Roman Empires great engineering accomplishments were introduced during the reign of the Caesars. The tradition continued under Vespasian, builder of the Coliseum, Trajan, builder of the Forum, and Hadrian, builder and possibly the designer of the Pantheon. Finally, a decade later Caracalla built a bath complex/recreation center in an effort to secure his own reputation in history.