The various buildings connected with an Irish monastery were generally but not always surrounded by a circular or oval rampart, which was at once a protection against enemies, or wild beasts, and also a limit beyond which the brethren were not allowed to wander without permission, and within which strangers, as a rule, were not allowed to intrude. Women were in all cases excluded from the sanctuary within this boundary. The wall or rampart was composed sometimes of earth dug up from a fosse at its base, when it was called a _rath_ or _lis_; sometimes of stone, when it was called a _caiseal_, and sometimes of earth faced with stone, and then it was known rather as a _caithir_ than a _caiseal_. The name _dun_, according to Dr. Petrie, was indifferently applied to any of these structures. But O'Curry quotes an ancient legal tract, which proves that the _dun_, strictly speaking, was "an enclosure made by two walls or mounds, with water between them." (_Manners and Customs_, vol. ii., p. 4.) This mur or mound was sometimes very strong and very high, fenced, too, with stakes on the top, and when necessary was double or threefold, with a deep dyke between each rampart. There was generally only one entrance, and when danger was apprehended from lawless foes, this entrance was strictly guarded night and day. It was considered sufficiently effective against the passing attacks of the native spoilers; but when the Danes began their bloody and relentless raids, the round tower was found to afford a much stronger and safer asylum. The monks in surrounding the ecclesiastical village with a _rath_ or _caiseal_, adopted no new contrivance. It was the custom of the country to surround the home of every chieftain's family with a similar defence, which the unsettled state of the country at the time rendered very necessary.