The rectangular plan, though excellent in a plain, was not always available in a hilly country. When the spot to be fortified was situate upon a height, the Egyptian engineers knew perfectly well how to adapt their lines of defence to the nature of the site. At Kom Ombo (fig. 33) the walls exactly followed the outline of the isolated mound on which the town was perched, and presented towards the east a front bristling with irregular projections, the style of which roughly resembles our modern bastions. At Kûmmeh and Semneh, in Nubia, where the Nile rushes over the rocks of the second cataract, the engineering arrangements are very ingenious, and display much real skill. Ûsertesen III. had fixed on this pass as the frontier of Egypt, and the fortresses which he there constructed were intended to bar the water-way against the vessels of the neighbouring negro tribes. At Kûmmeh, on the right bank, the position was naturally strong (fig. 34). Upon a rocky height surrounded by precipices was planned an irregular square measuring about 200 feet each way. Two elongated bastions, one on the north-east and the other on the south-east, guarded respectively the path leading to the gate, and the course of the river. The covering wall stood thirteen feet high, and closely followed the line of the main wall, except at the north and south corners, where it formed two bastion-like projections. At Semneh, on the opposite bank, the site was less favourable. The east side was protected by a belt of cliffs going sheer down to the water's edge; but the three other sides were well- nigh open (fig. 35). A straight wall, about fifty feet in height, carried along the cliffs on the side next the river; but the walls looking towards the plain rose to eighty feet, and bristled with bastion-like projections (A.B.) jutting out for a distance of fifty feet from the curtain wall, measuring thirty feet thick at the base and thirteen feet at the top, and irregularly spaced, according to the requirements of the defence. These spurs, which are not battlemented, served in place of towers. They added to the strength of the walls, protected the walk round the top, and enabled the besieged to direct a flank attack against the enemy if any attempt were made upon the wall of circuit. The intervals between these spurs are accurately calculated as to distance, in order that the archers should be able to sweep the intervening ground with their arrows. Curtains and salients are alike built of crude brick, with beams bedded horizontally in the mass. The outer face is in two parts, the lower division being nearly vertical, and the upper one inclined at an angle of about seventy degrees, which made scaling very difficult, if not impossible. The whole of the ground enclosed by the wall of circuit was filled in to nearly the level of the ramparts (fig. 36). Externally, the covering wall of stone was separated from the body of the fortress by a dry ditch, some 100 to 130 feet in width.