The book is dedicated to one of the largest archaeological monuments of the Crimean Mountains - the 'cave city' of Eski-Kermen, which arose in the Middle Ages. Eski-Kermen has been the subject of many articles in scientific journals, as well as separate chapters in scientific monographs and guides to the 'cave cities' of Crimea. However, a book entirely dedicated to Eski-Kermen had not existed before the publication of this monograph. While working on the book, the author repeatedly visited other 'cave cities' of Crimea, became acquainted with the antiquities of Asia Minor, in particular, with the Cappadocian underground cities.