After a long period of evolution, ancient odeons evolved into small roofed theatres, with characteristic semicircular shape and very high walls for the incorporation of a perimeter with large openings for lighting. Herodion Theatre at the foot of the Acropolis in Athens is such an extreme case, with 6.000 seats, it was similar in size with the large ancient theaters (the upper five rows of marble seats are not included in the modern reconstruction). Its construction (160-169 AD) necessitated the expropriation of four surrounding city blocks and required the solution of complex architectural problems. Its coverage by a single roof without intermediate supporting columns, even though mentioned by ancient sources, was considered by most scientists unfeasible. The book contains, in its second part, a series of observations on the building which indicate that the Herodion did indeed have a roof. It also contains its general design an evidence-based reconstruction of the roof, along with data on ancient large-scale construction. Finally, in its first part, the book includes an illustrated narrative of the study and execution of this most impressive work.