The essential uniformity of types which prevails over the immense area covered by the ancient pueblo ruins is a noteworthy feature, and any system of classification which does not take it into account must be considered as only tentative. What elements should be considered and what weight assigned to each in preparing a scheme of classification is yet to be determined, but probably one of the most important elements is the character of the site occupied, with reference to its convenience and defensibility. There are great differences in kind between the great valley pueblos, located without reference to defense and depending for security on their size and the number of their population, of which Zuni and Taos are examples, and the villages which are located on high mesas and projecting tongues of rock; in other words, on defensive sites where reliance for security was placed on the character of the site occupied, such as the Tusayan villages of today. Within each of these classes there are varieties, and there are also secondary types which pertain sometimes to one, sometimes to the other, and sometimes to both. Such are the cliff ruins, the cavate lodges, and the single house remains. The unit of pueblo architecture is the single cell, and in its development the highest point reached is the aggregation of a great number of such cells into one or more clusters, either connected with or adjacent to each other. These cells were all the same, or essentially so; for while differentiation in use or function had been or was being developed at the time of the Spanish conquest, differentiation in form had not been reached. The kiva, of circular or rectangular shape, is a survival and not a development.