 Complete the following phrase:  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 ByzantineSasanian War of 602628. 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 