The elaborate substructures of the podium were different on each side of the temple complex, as can be seen on the above plan, which sketches in their extant remains. The north front was supported by a series of vaulted chambers. In a number of these chambers the walls show incrustations of lime, evidence that there were water pipes and water storage here; there are traces of building and terracing lower on the slope. Imagine this whole slope covered with architecture, stairs, and fountains. One fountain piece, a ship's prow decorated with sea monsters and a boar's head, survives (for purposes of comparison, see this simple fountain from Pompeii).
The eastern front of the podium is well-preserved. Listen to the splashing of water. Richardson (A New Topographical Dictionary of Ancient Rome, 1992) describes this face as ornamented with niches alternately semicircular and rectangular, symmetrically disposed to either side of a larger central exedra. Between each pair of these niches are three or four semicircular niches at much smaller scale. Vaulted corridors and well-like chambers appear behind this facade in a complicated and irregular pattern, but they show no sign of having been used as reservoirs or for aquatic effects. In front of these niches ran a portico, either arched or columnar, and in its vaulted roof appear remains of a water channel. Apparently here water fell behind an architectural facade in nymphaea and was then channeled out through this to the north (this image shows a nymphaeum from the House of the Large Fountain in Pompeii).
The south front shows vaulted chambers at the extremities; the rest was evidently supported only by retaining walls. Off this front opened a number of subsidiary annexes of uncertain use.
The west front of the terrace was faced with arcades in two storeys built of massive blocks of travertine, heavily rusticated. The lower storey carried flat arches, and the upper carried rounded ones; see this nineteenth-century photo of the surviving remains and this impressive drawing by the eighteenth-century artist Piranesi (enlarge the pop-up window). The vaulted substructures behind these arches, rooms in brick-faced concrete masonry, were used as shops (Richardson, 88).
Temple of Deified Claudius