The city was greatly expanded and deepened in the Eastern Roman (Byzantine) era, when it was used for protection from Muslim Arab raids during the four centuries of Arab–Byzantine wars (780-1180). The city was connected with Derinkuyu underground city through miles of tunnels. Some artifacts discovered in these underground settlements belong to the Middle Byzantine Period, between the 5th and the 10th centuries A.D. These cities continued to be used by the Christian inhabitants as protection from the Mongolian incursions of Timur in the 14th century.