The openings have inclined jambs, being 1 foot 8 inches at base and 1 foot 5 inches at top, and the height from the sill to the springing is 4 feet 9 inches. Each window has two orders on the exterior--a plain inner order and an outer order, containing a shaft wrought on the jamb angle. The inner order has plain square-edged jambs and round arch. The outer order has also a square-edged round arch, springing from the caps of the shafts. This arch, however, is not concentric with the inner arch, but is carried considerably above it, leaving a flat space or shield between the arches. The caps and bases of the shafts are now much decayed, but they have a distinctly Norman character. Above the highest string course the wall rises about two feet, so as to form a parapet. The diameter within this wall is 8 feet 8 inches, and the wall is about 2 feet 6 inches thick. The top of the wall is covered with broad stones, each the full width, which have an ogee moulding wrought on the outer edge. The masonry of the tower is constructed in regular courses of ashlar work, carefully wrought to the circle. The lower twelve courses are of a hard grey freestone, and the portion above consists of a yellowish freestone of similar workmanship, which is softer, and has decayed considerably both on the exterior and interior. This difference in the materials of the lower and upper portions has led to the belief that the upper part has been rebuilt at a later date than the lower part. [Illustration: FIG. 114.--Abernethy Tower. View from North-East.] The tower is slightly less in diameter at the top than at the bottom, but it tapers so little that the difference is scarcely observable. In this tower we find features which belong both to the Celtic and Norman styles of building. The circular form of the tower and the disposition of its parts, the door raised some feet above the ground, the small windows in the lower stories, the four large windows in the top story, and the inclining jambs of door and windows, all point to the Irish origin of the design; while the distinctly Norman character of the details of the four upper windows, and the regular coursing and dressing of the masonry, show the influence of the Continental style of the twelfth century. RESTENNET PRIORY, FORFARSHIRE. The remaining towers of the group above enumerated are all square on plan, and certain of them are connected with churches, and form a part of them. In these respects, therefore, they differ considerably from the towers of Abernethy and Brechin. They were all probably built during the twelfth century. Possibly Restennet is an exception. It is the only one of the square towers which has very marked features of a pre-Norman character. The outside dimensions of the Tower of Restennet (Fig. 145) are 15 feet 11 inches from north to south by 15 feet 6 inches from east to west, and the walls are 2 feet 8 inches in thickness.