Diocletian was the first emperor in the history of the empire to retire voluntarily, doing so to his palace at Split in Croatia. He could do this because of the stable system of government called the Tetrarchy he put in place. Under this system, the empire was ruled by two senior emperors, one in the east and one in the west, each with a junior to assist them. He also reformed the coinage, passed edicts controlling prices and strengthened the military. Later Christian writers were however critical of him for his persecution of Christians.