In one or two architectural features Cliff Palace is unique, although sharing with other cliff-houses of the Mesa Verde National Park many minor characters. The first difference between Cliff Palace and Spruce-tree House, outside of the disparity in their size and the relatively large proportion of secular to ceremonial rooms in the latter, is the existence in the former of terraces and retaining walls. Spruce-tree House is built on one level, above which rise the secular houses while below are the ceremonial rooms or kivas. The contrast of this simple condition with that of Cliff Palace, with its three terraces and the complicated front wall at several levels thereby necessitated, is apparent. There are several other ruins in the Mesa Verde Park in which the configuration of the rear of the cave led to the construction of the cliff-house in terrace form. This is well exemplified in the Spring House, where buildings on an upper level occupy much the same relation to those below as the ledge houses to the main ruin, and in ruins in the Canyon de Chelly, like those in Mummy Cave, where this relation of the buildings on the ledge to those on top of the talus is even more pronounced. Architectural features in cliff-houses are due to the geological structure of the cave in which they are situated rather than to cultural differences. Nothing was found to indicate that Cliff Palace was inhabited during the historic period. The inhabitants were not acquainted with metals brought by white men to the Southwest. The absence of glass and of glazed pottery is also significant. No sheep, horses, or other beasts of burden paid them tribute. In fact, there is no evidence that they had ever heard of white men. These ruins belong to the stone age in America and show no evidence of white man's culture. Except that it is prehistoric, the period at which Cliff Palace was inhabited is therefore largely a matter for archeological investigation to determine, and thus far no decisive evidence bearing on that point has been produced. It has been held that Cliff Palace is five hundred years old, and some writers have added five centuries to this guess; but the nature of the evidence on which this extreme antiquity is ascribed to the ruin is not warranted by the evidence available.