The walls of Cliff Palace present the finest masonry known to any cliff-dwelling and among the best stonework in prehistoric ruins north of Mexico. A majority of the stones used in the construction were well dressed before laying and smoothed after they were set in the wall. The joints are often broken, but it is rare to find intersecting walls or corners bonded. Stones of approximately the same size are employed, thereby making the courses, as a rule, level. Although commonly the foundations are composed of the largest stones, this is not an invariable rule, often larger stones being laid above smaller ones; the latter, even when used for foundations, are sometimes set on edge. As a rule, the walls are not plumb or straight. The custom of laying stone foundations on wooden beams is shown in several instances, especially in cases where it was necessary to bridge the intervals between projecting rocks. The arch was unknown to the masons of Cliff Palace; there are no pillars to support floors or roofs as in Spruce-tree House. It is not rare, especially in the kivas, to find instances of double or reenforced walls which may or may not be bonded by connecting stones. The masonry of the kivas as a rule is superior to that of the secular rooms. The mortar employed in the construction is hard; the joints are chinked with spalls, fragments of pottery, or clay balls. The fact that much more mortar than was necessary was employed resulted in weakening the walls. Several walls were laid without mortar; in some of these the joints were pointed, in others not.[23] The ancient builders did not always seek solid bases for foundations, but built their walls in several instances on ashes or sand, evidently not knowing when the foundations were laid that other stories would later be constructed upon them. [Footnote 23: Fragments of mortar from the walls and floors, ground to powder, were used in the repair work.] In several sections of the ruin there are evidences that old walls, apparently of houses formerly used, served in part as walls for new buildings. There are also several instances of secondary construction in which old entrances are walled up or even buried and old passageways covered with new structures. Similar reconstruction is common in Hopi pueblos, where it has led to enlargement of rooms and other variations in form. Among the several examples of such secondary building in Cliff Palace may be mentioned a long wall, evidently the front of a large building, which serves as a rear wall of several rooms arranged side by side. The obvious explanation of such a condition is that the walls of the small rooms are of later construction.