The so-called Twin Towers, which seen together from certain points appear as one ruin (pl. 15, _a_, _b_), rank among the most impressive buildings in Square Tower Canyon. They stand on the south side of the canyon on a rock isolated by a cleft from the adjoining cliff. The larger (fig. 9, _A-E_) has an oval ground plan and a doorway in the southwest corner; the smaller (_F_, _G_, _H_, _I_) is horseshoe shaped with a doorway in the east wall, which is straight. The arrangement of rooms is seen in figure 9. Small walled-up caves are found below the foundation on the northwest base of the larger room. [Illustration: FIG. 9.—Ground plan of Twin Towers.] RUIN 9 The ground plan of this ruin is rectangular in form, 19 feet 6 inches long by 10 feet wide. The standing walls measure 11 feet in altitude. It is situated on the south rim at the mouth of the South Fork, just above Ruin 7, a few feet back from the cliff. A doorway opening in the middle of its north wall was formerly made difficult of entrance by walls, now fallen, extending from the northeast and northwest angles to the edge of the cliff. The masonry throughout is rough; projecting ends of rafters indicate a building two stories high. There are peepholes with plastered surfaces through the southeast and west walls, which suggest ports. A short distance east of the building is a circle of stones reminding the author of a shrine. UNIT TYPE HOUSE (RUIN 10) This pueblo (pl. 19, _c_), from a comparative point of view, is one of the most interesting ruins in the Hovenweep, and is situated on the very edge of the canyon on the North Fork not far from where it begins. It is the simplest form of prehistoric pueblo, or the unit[47] of a pure type, made up of a centrally placed circular ceremonial room (fig. 10, _K_) embedded in rectangular rooms, six in number (_A-F_). The resulting or external form is rectangular, oriented about due north and south; the southern side, which formerly rose from the edge of the canyon, being much broken down and its masonry precipitated over the cliff. [47] The “unit type” was first recognized by Doctor Prudden in his illuminating studies of the pueblos of the San Juan Basin. The author was the first to point out its existence in cliff-houses of the same area. The central kiva (fig. 10) is made of exceptionally fine masonry and shows by what remains that it had mural banquettes, and pilasters to support the roof, with other features like a typical kiva of the Mesa Verde cliff-houses. A side entrance opens in one corner into a small room (fig. 10, _G_) in which ceremonial objects may have been formerly stored (pl. 32, _b_).