The book offers a detailed study of the architecture of the Caseggiato del Serapide in Ostia Antica and of the insula III, X, 1-2-3, starting with an analysis of the urban layout of the settlement from the first half of the 2nd century AD to the Severan age, when the city was mostly made up of brick buildings and presented an image very similar to Rome. The description of the Caseggiato del Serapide, built during the reign of Hadrian, sheds light on the various construction phases and outlines the compositional side of an intervention that gradually filled the block, starting from a pre-existing trajanic building in the centre of the lot. Various portions of the building, floors and masonry are examined from a technical point of view, elaborating hypotheses on the consistency of the upper floors based on the stresses to which the structural elements were subjected. Together with the Caseggiato del Serapide, a rather singular building due to the shape of the pillars that characterise its courtyard, the unusual curvilinear facade that would have marked the side elevation of the Caseggiato in the Hadrianic phase is also described, certainly the most interesting element of the building complex in the early phase of its history.