Hill-forts, the refuges and strongholds of the early inhabitants, exist in every country of Europe. Their range in time extends from the early prehistoric through the early historic periods of the racial areas in which they are found. They have no more definite form than that of a prevailing but irregular circularity. The site selected is usually enclosed and fortified with due regard to its specialities of situation and defensibility. Sometimes the fort instead of occupying the whole hill-top may occupy only the most defensible part of it. In other cases the whole eminence may be surrounded by defensive constructions completely encircling and protecting its upper portion. Occasionally these forts, though situated among the hills, are planted in the lower ground, commanding an extent of meadow-land or pasture. With regard to their construction, the hill-forts are usually either earthworks or stoneworks, rarely a mixture of both. In France the Gaulish forts of the pre-Roman period were often such extensive works as to be termed oppida by the invading Romans. Though built of dry-stone masonry, the parts of the walls most exposed to attack were bound together by great logs of wood, placed both longitudinally and transversely within the thickness of the rampart, so as to resist as much as possible the assaults of the battering-ram. The great dry-built stone rampart of the prehistoric fort at Burghead, in Elginshire, is similarly strengthened by logs of oak, but it is the only example of this method of construction yet known in Scotland, where hill-forts are perhaps more numerous than in any other European country. They are generally called 'duns' (see DUN) in the northern and 'camps' in the southern districts, where the older term survives in connection with a number of the principal forts, as Dum-barton (Dun Bhreatain), Dundonald in Ayrshire, and Dunpelder in Lothian, not to mention Dun Edin as the old name of Edinburgh. Among the most remarkable of the hill-forts of Scotland may be mentioned those of the two Caterthuns in Forfarshire—one a good example of the fort with earthen rampart, and the other with walls of dry stone—the Tap o' Noth, and the twin-summits of Benachie, each with its massive fortifications of stone, in Aberdeenshire, the remarkable stone fort of Dun Tuathal on Drummond Hill, overlooking the junction of the waters of the Lyon and the Tay, and one equally remarkable, called Dun-dalamh, in a similar situation in Laggan on the Spey, Inverness-shire.
Many of the dry-stone forts in Scotland present the peculiar feature of a partial vitrification of the materials of their walls. The same thing has been observed in connection with similar forts in Ireland, France, and Hungary. The attempt to account for the existence of this peculiarity has given rise to much speculation and controversy. But it seems to be clearly established that the so-called vitrified forts do not differ from the other dry-stone forts, if the vitrification be not regarded as a process of construction. No relevant and conclusive evidence on this point has been obtained from examination of the structures themselves; and against the arguments in support of the view that the vitrification was intended as a cementing process we have to put the facts (1) that no fort is wholly vitrified; (2) that where vitrification exists it occurs in patches, affecting sometimes a portion only of the thickness of the wall; and (3) that when it occurs on the exterior surface of the wall the upper parts are sometimes found partially vitrified, but with no trace of vitrification on the portions underneath. Among the best known of the so-called vitrified forts in Scotland are the Tap o' Noth in Aberdeenshire, Craig Phadric and Dunbhairdgall in Inverness-shire, Knockfarril in Ross-shire, Dun Mac Uisneachan in Argyllshire, and Finhaven in Forfarshire. In Wales stone forts are most numerous, while in England earthworks predominate. The earthen forts of Sussex explored by Colonel Lane Fox are sometimes of considerable magnitude, that of Cissbury, for instance, enclosing a space of 60 acres. They are, as a rule, of prehistoric origin. Some of the stone forts of Ireland, especially those of the Aran Isles, are of great magnitude and well preserved. Photographic views of them are given in Lord Dunraven's book on Early Irish Architecture.
Consult also Dr Christison's 'Prehistoric Forts of Peebles-shire,' and 'The Duns and Forts of Lorne' in the Proceedings of the Society of Antiquaries of Scotland (vols. xxi. and xxiii.); 'Mémoires sur les Ouvrages de Fortifications Gauloises,' &c. in the Compte Rendu du Congrès Archéologique de France, at Toulouse in 1874 (p. 427); 'Les Camps Barbares fortifiés en Hongrie,' by F. F. Romer, in the Compte Rendu of the Congress of Prehistoric Archaeology held at Budapest in 1876 (vol. ii. p. 68); and 'Helvetische Denkmäler,' by Dr F. Keller, in Mittheilungen der Antiquarischen Gesellschaft in Zürich (vol. xvi.).