In Ireland every king had his dun. This was an enlarged rath with an outer court in which he held his hostages, for the law required this: "He is no king who has not hostages in lock-up."
Dun in Welsh is din, and dinas is but another form of the same word, and signifies a royal residence.
A gloss to an old Irish law tract says that a royal dun must have two walls and a moat for water.
Dun in Scotland is applied to any fort. According to the Gaelic dictionaries, it is "a heap or mound," and even a dung-hill is a dun.
In fact, the French dune and the Cornish towan derive from the same root. Dun so much resembles the Anglo-Saxon tun that we cannot always be sure of the derivation of a place-name that ends in tun.
Every tribe had its dun, to which the cattle were driven, and where the women and children were placed in security in times of danger. This would be in addition to the royal residence, that is the dun of the rig.
Within the dun were numerous structures of timber, roofed with oak shingles, some of a large description, such as a banqueting hall; but the habitations of the garrison were circular, of wicker-work, and thatched with rushes.
In Cornwall there is Dingerrein, the dinas of S. Geraint; Castel-an-Dinas; Damelioc (Din-Maeloc); Dunheved, the old name for Launceston; Dundagel.
4. I come now to the stone fortresses that are found in parts of Cornwall and Wales. They are also to be seen in Scotland and Ireland. These are called caerau in Wales. A cathair is the term applied to them in Ireland, and cathair signifies as well a city.
They are found in England only in Somersetshire, Devon, and Cornwall; and in Wales only in such parts as were invaded and occupied from Ireland.