From their similarity in external shape and distribution, circular ruins and round towers have been regarded as in some way connected. It by no means follows that rooms inside their external walls were identical in use. For instance, the so-called Great Tower on the cliffs overlooking the San Juan, described and figured by Prof. Holmes, is said by him to measure 140 feet in diameter, and to have double walls connected by partitions, forming a series of encircling rooms. This ruin may be classified not as a tower but a circular ruin, and the same may be said of the so-called Triple-wall Tower, rising on the border of rectangular rooms, situated at the mouth of the McElmo. The dimensions of this so-called tower are reported to be “almost” the same as the Great Tower. The author regards these as examples of an architectural type related to towers, from which it is distinguished not only by size, but also, especially, by the arrangement of rooms on their peripheries. The internal structure of the tower type is little known, but in none of these buildings has the author detected peripheral rooms separated by radial partitions, although one of these radial partitions is found in kiva A of Sun Temple. The original building of the last mentioned ruin, although D-shaped, has a morphological similarity in the arrangement of peripheral rooms to the “Great Tower” of the San Juan, or that on the alluvial flat in the Mancos, and the “Triple-wall Tower” room of the McElmo, save that the so-called innermost of the triple walls is replaced in Sun Temple by two circular walls, side by side, forming kivas B and C. The tower, with annexed rectangular rooms, like its homologue, the circular kiva with similar adjacent chambers surrounding it, is practically the “unit type,” a stage of pueblo development pointed out by Doctor Prudden,[22] who does not make as much as would the author of the intra-mural condition of the kiva, or its compact union with domiciliary rooms. Far View House on the Mesa Verde is a good example of this union of form, characteristic of the “unit type” or compact pueblo with embedded circular kivas, one of which is central, probably the first constructed, and of large size. Such compact pueblos are numerous on the Mesa Verde, judging from central depressions in mounds, and characteristic of the San Juan, at least of its northern tributaries. The previous stage in pueblo development is that in which the sanctuary or tower (kiva) and habitation are distinct. The extra-mural circular kiva,[23] or circular room separated from the house masses either in courts, as in Rectangular and Round villages, or situated outside the same as in “Line villages,” like Walpi, or pyramidal forms, is like Zuñi or Taos and more modern pueblos. This modification is widely distributed in ruins south of the San Juan, still persisting in several modern pueblos.