There still remain some special examples of Celtic structures to be mentioned. These are the well-known round towers, of which those at Abernethy and Brechin have already been referred to. A third round tower is also found attached to the church of Egilsay, in Orkney.[45] These towers are, undoubtedly, all examples of a style imported from Ireland. They are detached specimens of a group, of which no fewer than seventy-six examples still exist in that country, besides twenty-two others which are known to have existed formerly. It has been shown by Dr. Petrie that the Irish round towers were erected as places of refuge in connection with monasteries, to which the monks might repair with their relics and treasures in case of alarm. Such shelter was only too much required, as the valuables of the monastic institutions formed a very tempting bait for pillage by the Norsemen, whose depredations were so alarming during the ninth century. The history of the round towers of Ireland is easily traced in their architecture, and has been fully explained and illustrated by Dr. Petrie in his well-known book on the subject, and in the late Lord Dunraven’s beautiful work on the early structures of Ireland. These towers are always found associated with religious sites. The earliest examples are comparatively rude in structure, while the later ones gradually improve in style of masonry and finish, until the latest are built with ashlar work, and contain some Romanesque ornaments and details. In all, however, the leading principles of their construction are the same. (Fig. 12.) The tower is round on plan, and is finished on top with a conical roof. The door is narrow, and is placed, for security, at a considerable height above the ground, and the lower floor is sometimes built up solid, so as to resist conflagration. The windows are small, except those on the top story, which are generally set facing the cardinal points, and are larger, so as to allow the sound of the bell to be heard--one of the uses of the tower being to serve as a belfry. The Irish practice of inclining the jambs of the doors is maintained, and in the early examples the lintel is straight, while in the later ones the door is finished with a semi-circular arch, and enriched with several orders of mouldings and ornaments bearing a markedly Norman character. This remark applies also to the four windows of the top story, which are plain in the early examples, and gradually become more ornamental and Norman like. The Irish towers are almost invariably built alone, and free from other structures; but some late examples are constructed in connection with churches, and enter from them by a door on the level of the floor of the church. The idea of using these towers as a place of security is thus departed from, and they are then simply of use as belfries.[46] Finally, they become absorbed into the structure of the church, and are erected merely to serve as belfries on the gable.