Starting in the early 3rd century, matters changed. The "
Crisis of the third century" defines the disasters and political troubles for the Empire, which nearly collapsed. The new feeling of danger and the menace of barbarian invasions was clearly shown by the decision of Emperor
Aurelian, who at year 273 finished encircling the capital itself with a massive
wall which had a perimeter that measured close to 20 km (12 mi). Rome formally remained capital of the
empire, but emperors spent less and less time there. At the end of 3rd century
Diocletian's political reforms, Rome was deprived of its traditional role of administrative capital of the Empire. Later,
western emperors ruled from
Milan or
Ravenna, or cities in
Gaul. In 330,
Constantine I established a second capital at
Constantinople.