As the Roman Empire began to decline, so did Milan; but it regained prominence under Carolingian rule during the 8th century. Milan became autonomous, trade increased enormously and the city grew in size, influence and wealth. A new city wall, featuring six gateways, was built for fortification. A few of the gates can still be seen today. The city also developed a canal system, designed to assist in defence of the city and also for agricultural purposes.