In this era the wall of the castle left from the huriis was recruited. The city’s main roads were shaped in this era. The first step of construction of homes of administration class, and bazaar places used by public left outside the castle, were first done in this era. Romans, who had dominance in the city since 69 B.C. had protected the city and built temples with big yards. Water channels, left from the Helen era, were repaired and recruited in this era. Spreading of Christianity in Diyarbakir had occurred in this era as well. After taking approval in 313 A.C. with Milano rescript, Christianity, religion of the Emperor, began spreading easily and structures such as; monastery, churches, and libraries were built in city settling. Since the middle of the IV. Century, the Romans, turned the city into Roman Mesopotamia’s capital, Constantius the second had repaired the castle and built a “Darissanaa” for the production of war machines. Constantius the 2nd had built outer walls in a semi-circular shape as to cover the inner castle as well, in 349. The walls were enlarged between 367-375 in a way to include the community and today’s Mountain Door -Urfa Door-Mardin Door walls were built and the old west walls, today’s Gazi Avenue, were destructed. According to resources, the doors were locked every night with the sunset and the keys were taken to the palace. This rule was so strict that, no traveler would be accepted in without the permission from the pasha through their consul. The main roads intersecting in the city are in the qualification of a plan. Due to intensity of trade activities, there has also been a bazaar street in the center. The construction of outer walls in IV century, the great immigration to the city, and Roma using Diyarbakir as military center, has had great effects on the development and shaping of the city.