The position chosen for this city was one of considerable natural strength. Its walls surround the broad top of an outlying hill which is connected with the watershed lying to the south only by the high ground in that direction. On either side it is cut off by the steep valleys of two mountain-streams flowing northward, which meet just below the modern village. These in turn are fed by small tributaries from just behind the hill, which is thus almost enclosed. From the point where these rise the fall is about a thousand feet to the confluence of the main streams two miles away; and though the descent of the latter is necessarily more gradual, they are still very rapid, and in the winter are foaming torrents. That on the eastern side in particular, the Beuyuk Kayanin, has by its force worn down its rocky bed so deeply that where it passes by the eastern knoll of the citadel, called Beuyuk Kaleh, its banks have become precipitous cliffs requiring little or no artificial defence.[478] The Yazîr Daresi, on the western side, flows through more alluvial ground, and has there scooped for itself a gorge, in the steep bank of which the harder rocks are left protruding, thus rendering an assault uninviting on that side also. The engineers who planned the defence utilised the natural advantages of the position, banking up the slopes, and bringing their wall wherever practicable to the edges of the rocks, in which all possible footholds were filled up with masonry. [Illustration: PLATE LVIII BOGHAZ-KEUI: SITE OF PTERIA, THE NORTHERN CAPITAL OF THE HITTITES Beuyuk Kaleh. Sary Kaleh. Yenije Kaleh. The Maiden’s Rock. B. Kayanin Daresi. The Lower Palace. The Acropolis covered the whole hill; the line of ancient ramparts forms the horizon on the right. (_See pp. 32, 200._)] On the north side, where the line of defence is less clear, the ground is broken by a third small stream, the Kizlar Kaya Daresi, which rises within the circuit of the wall in the high ground of the acropolis, and now joins the Yazîr in the modern village at its foot. On the level ground, near this junction, there are the traces of an ancient rampart; but the line of natural defence being somewhat higher, it may reasonably be suspected that the enclosure was at some time extended in this direction, possibly in order to include the Lower Palace. However that may be, the really vulnerable point would seem to have been by way of the higher ground to the south, and here the artificial protection was stronger in proportion. The wall seems to have been built on this side upon a rampart revetted with stone, which in its turn followed the line of a natural ridge in the ground, giving an almost impregnable appearance to the enormous mass of the defensive works. So high is this mound that a narrow subterranean way was constructed through it, giving access to the interior.