The exterior walls of the chapel (Fig. 192) form an oblong, measuring 31 feet 7 inches from east to west, by 16 feet in width at the east end and 14 feet 4 inches at the west end. The angles of the structure are thus not right angles. Although the building is extremely simple in its external aspect, it is more ornamental and of a more developed form of plan in the [Illustration: FIG. 193.--St. Margaret’s Chapel, Edinburgh Castle. Elevations.] interior. The plan comprises a nave with a chancel arch, and a chancel which has a round apse formed within the square east end of the exterior. The nave is 15 feet 10 inches long by 10 feet 4 inches and 9 feet 9 inches wide at the east and west ends respectively. The chancel is 9 feet 7½ inches long, and the same width at the west end. The total interior length is 27 feet 4½ inches. The nave is waggon vaulted, without ribs; and the apse is roofed with a vault forming the quadrant of a dome. The nave vault is about 10 feet 8 inches high. (See sections, Fig. 192.) The chapel stands on the extreme summit of the Castle rock, and the rock which is seen rising against it on all sides, except the north (on which side the level outside is nearly the same as that of the floor), has, it is understood, in recent years been partially cut away on the south, east, and west sides. (Fig. 193.) This operation has to be kept in view in considering the masonry of the exterior walls, for on those three sides there has necessarily been a certain amount of underbuilding, which is consequently of a much more modern character than Norman masonry. [Illustration: FIG. 194.--St. Margaret’s Chapel, Edinburgh Castle. View from South-West.] The genuine surviving Norman masonry begins below the line of the south windows (see Fig. 194). The first two or three courses, which extend up to the sills of the south windows, are built of reddish stone, above which are five courses of a bluish grey stone, the upper course being above the windows. All the masonry above that level on the south and north sides is later work. The masonry of the north and east sides has been greatly interfered with, as has also that of the west end, where, however, there are several consecutive Norman courses. The east wall appears to have been carried to a greater height than the other walls (Fig. 194), as if some higher building had been attached to the chapel. The alterations above referred to show that the side and end walls have been raised, and we have no doubt that the upper part of the west gable above the window, with the ridge course, is modern, or at least of a date long subsequent to the original building. The present porch in front of the entrance door on the north side, near the west end, is quite modern, although the doorway itself is ancient. A [Illustration: FIG. 195.--St. Margaret’s Chapel, Edinburgh Castle. Elevation of Chancel Arch.]