In Cappadocia, as in many different parts of the world, there are numerous underground defense structures. The volcanic tuff, which is easy to excavate and spreads over hundreds of square kilometers, has allowed the excavation of many underground defense structures in this region. So, in almost every village of Cappadocia, there is at least one, possibly more than one, underground city. Although some of these structures -which are scattered over such a large area and of which only a small number have been explored- are small, singular structures, some other underground cities are thousands of meters long and have countless rooms. In this chapter, four different underground cities in Koramaz Valley are described, after giving an overview of the historical background of these defense structures in Cappadocia, discussions about their first construction dates, and the great examples that have been researched so far. Apart from these four big defense structures, there are also numerous small underground cities in the valley.