The general plan is circular and the construction simple. A deep ditch or trench was dug and the earth thrown up, forming a steep enclosing dyke or rampart, which was probably rendered further secure by a stake fence. They vary in size from a few yards to a hundred in diameter, and often consist merely of the circular entrenchment, the area of which is slightly raised above the level of the adjoining land. These are common in the central parts of Ireland, such as the motes of Granard, Slane, and Clones. But in the case of many of the more important they were enclosed by two, three, or more breastworks; and within the inner rampart stood a steep mound, flat at the top, on which was erected the house of the king or chief, overlooking the camp. On the verge of steep cliffs, or rocky headlands jutting into the sea, and on the summit of inland hills, these defences were generally erected. Sites were chosen whose natural features afforded a ready means of rendering the positions secure. The term _Dun_ is usually applied to this class of fortification, and it enters into the composition of many place-names, Dr. Joyce stating that it is used as a prefix to 600 townlands. Alignments of forts are an important feature in Ireland, and will be found especially on the sea-coast from Waterford, westwards, and round to the north of Mayo, and also in many inland districts. Perhaps the largest earthwork in Ireland is the Dun at Dorsey, County Armagh. It is about one mile long and 600 yards wide, and consists of a great embankment with a fosse on each side and outer ramparts. Portions still remain sufficient to show the magnitude of this great earthwork when originally constructed.[77] * * * * *