The initial Christian settlement of the region came in the first centuries AD, starting with hermits who retreated into the seclusion of the tuff landscape from the Christian community at Caesarea. They either settled in caves that already existed or dug their own residences in the cliffs. Since they were seeking solitude rather than protection from enemies, they largely made their homes above ground level. After the Christian church was re-organised under the Cappadocian Fathers (Basil of Caesarea, Gregory of Nyssa, and Gregory of Nazianzus) in the fourth century AD, ever larger groups of Christians followed them over the next few centuries, settling in Cappadocia and building cloistered communities, which meant that they needed ever more and ever larger residential and religious spaces. Meanwhile, in the fourth century, the Isaurians invaded, in the fifth century the Huns, and finally in the sixth century the Sassanid Persians; in 605 the city of Caesarea was conquered during the Byzantine–Sasanian War of 602–628. These incursions sparked the intensive construction of rock-cut buildings below and above ground, including whole cities. The design of these structures was principally shaped by security and defensive concerns. From 642, the Arabs began to invade the region and these concerns grew increasingly significant, with the result that Christian communities continued to live in the region for three centuries, secure from raids. Eventually, the Byzantines regained control over Cappadocia and under their rule Christianity and Christian architecture in Cappadocia entered a golden age. By the eleventh century, roughly three thousand churches had been carved in the rocks.