_Brochs_ are also peculiar to Scotland. They are massive, tower-like buildings, chiefly occurring in the northern counties and upon the islands; they are remarkably similar in outline and construction, and they have been ascribed chronologically to the period immediately before or after the Roman occupation of Britain, and as being essentially Celtic. The Broch of Mousa is generally believed to be the most perfect example extant; it is in Shetland, and consists of a wall 15 feet thick enclosing a court 20 feet in diameter. The wall is about 45 feet in height and contains a solitary entrance, narrow and low. In the thickness of the wall, and approached by three internal openings, are chambers, while a spiral staircase leads upwards to where passages constructed in the walls are served by the stairway. Other Brochs which have been examined appear to possess a similarity of plan, but some have subsidiary defences in the shape of external walls, ramparts, and fosses; thus the example at Clickamin, Lerwick, was surrounded by a stone wall. That found upon Cockburn Law, and known as Odin's, or Edin's Hold, is of note by reason of the double rampart of earth surrounding it. It is one of the largest as yet discovered, the wall being 17 feet thick and the area 56 feet wide. Probably the many hut circles which surround this Broch are of later date and were formed from its ruins. The great thickness of the wall is exceeded, however, by the Broch at Torwoodlee, Selkirkshire, by 6 inches. With the advent of the historical period firmer ground is reached, and there are numerous evidences that the Motte and Bailey Castle was introduced at an early period into Scotland. During the second half of the eleventh century this was the prevailing type as in England. It has been found possible to divide the era of castellation proper in the northern kingdom into four distinct periods: _First Period_, 1100-1300.--The roving spirit and warlike disposition of the Normans prompted their adventurers to penetrate into the fastnesses of the North, where the innovations they introduced made them acceptable in the main to the inhabitants. They taught the latter how to raise towers of a design based upon the Rectangular Keep, with thick cemented walls, and many of the great fortresses, such as Edinburgh, Stirling, and Dumbarton, originated at this time. The early type of Keep was quadrangular in plan with towers at the angles, which were sometimes detached from the main building and placed upon short curtain walls; but some were naturally modified or specially adapted to the site like those of Home and Loch Doon. The use of water as a defence was recognised at an early stage; some towers were placed on islands in lakes, and most of them were furnished with moats and ditches. At this period castles were seldom placed upon high promontories. The workmanship was as a rule poor, rough, and crude, but some exceptions occur like Kildrummie and Dirleton.