Such is the careful plan of the complex, justifying this detailed treatment because it is a turning point in the history of Roman architecture, perhaps the most seminal architectural complex in the whole Roman world. Everything (Fig. 5.5) centers on an axis, everything rises, aspires to the apex at the cult-statue, embracing a superb and at each level more extensive view of the plain stretching away southward to the sea. The materials and technique with which this form is realized and supported are interesting in themselves and for what they contribute to the dating of the Sanctuary. The basic materials are tufa, limestone, and concrete; no marble is used except in statuary. Limestone, which in Roman architecture comes to predominance later than tufa, is used for the facing of polygonal walls and _opus incertum_, for décor (_e.g._, the Corinthian capitals of tufa columns), for pavements. The limestone spalls or chips left over from the facing of _opus incertum_ were used in concrete cores and for fill. Tufa is used for footings, structure in squared blocks (_e.g._, caissons for concrete), the voussoirs, or wedge-shaped blocks, of arches, column drums, the core of stuccoed decorative elements, cornices, corners. Both materials are subordinate to concrete. The use of concrete at Palestrina amounts to an architectural revolution, and, as often, the revolution in taste is combined with a revolution in materials and methods. This strong, cheap, immensely tough material enabled the architect to enclose space in any shape; henceforward architects could concentrate on interiors, and the day of the box-like temple was over. The architectural history that culminates in the Pantheon begins here. The architect was clearly more expert in the use of concrete than in the use of stone. Palestrina concrete is hydraulic, a combination of limestone chips and mortar made of _pozzolana_ (volcanic sand) and lime. Concrete footings, Fasolo and Gullini found, go down to bedrock everywhere; _e.g._, each of the three rows of columns of the Cortina Terrace portico rests on a foundation wall of concrete based on bedrock, while the space between is hollow, to relieve weight. For the same reason the whole hollow square of the Cortina Terrace rests on a series of rectangular concrete coffers with a stone fill. The result of this use of concrete is that the whole Upper Sanctuary is structurally a single unit. Each level is planned as a step toward, and a retaining wall of, the level next above. The stresses, Fasolo reports, are never more than about three pounds per square yard for walls and eight pounds per square yard for columns; this in a structure which is in effect a skyscraper 400 feet high. There is repetition of motif throughout, not from paucity of imagination, or because it is the easy way, but of set aesthetic purpose, to emphasize the concealed structural unity and to use the functional parts of the complex to give architectonic unity to the whole.