The abbey of San Sebastiano, which lies not far from the town of Alatri in Southern Lazio, preserves within its walls almost fifteen hundred years of history. This history is unusually bound to a number of important figures, from Saint Benedict to pope Nicholas V and his humanist court. For the past four years a small team has been investigating the standing structures of the abbey, reconstructing the stratigraphy of the standing walls and tracing the various phases through the building. The study has produced some startling discoveries: the oldest complete plan of a monastery in Europe, and one of the earliest Renaissance villas. The book gives an account of the architecture and the history of the building, showing how each phase relates to the last both structurally and thematically.