Hadrian and Constantine were among the best Roman emperors. Hadrian rebuilt structures like the Pantheon and constructed Hadrian's Wall. Constantine reunited the empire under one ruler and founded Constantinople. Commodus, Nero, and Caligula were among the worst. Commodus was a megalomaniac who took no interest in governing. Nero was also a megalomaniac obsessed with the arts who was blamed for the great fire of Rome. Caligula thought he was a god and was vain, cruel, and possibly mentally ill.