There are about 3,000 hill forts in England, concentrated in the south and west, with especially high numbers in the southwest peninsula (Devon and Cornwall have a total of 285 large hill forts). There are also about 570 hill forts in Wales. Although some originated in the Bronze Age, the general consensus is that the majority of hill forts in Britain were constructed during the Iron Age (between 700 B. C. and the Roman conquest of Britain in A. D. 43). By the 8th century B. C. only a few existed, but gradually the numbers increased and existing ones were strengthened. The further development seems to have occurred in the last century before the Roman conquest. In many cases, however, it is difficult to say which are prehistoric and which are of later periods, and this rough timetable applies for England only. Events in western and northern regions, as well as in Ireland, were probably delayed by a century or so, possibly more.
There are also variations from area to area in the size, type and density of sites. The largest numbers of the sites are in the western counties, namely in Dyfed and Cornwall with a majority of small univallate sites enclosing only one or two acres. Larger hill forts are found in southern England, the Cotswolds and the Welsh borderland. The Wessex region (Dorset, Wilshire, Hantshire and Berkshire) is one of the classic hill fort areas in Britain, with about 150 sites, a number of them very large and spectacular, including the most famous hill fort of all, Maiden Castle.
The Romans eventually occupied some of the hill forts, such as the military garrison at Hod Hill and Brean Down, but others were destroyed and forsaken. Some of the place names of these sites bear the suffix “-bury” (e. g., Cadbury), meaning fort. Where the Roman influence was less strongly felt, such as in uninvaded Ireland and in the unsubdued northern parts of Scotland, hill forts and other forms of fortified settlements were still built and used for several more centuries. Some hill forts were reoccupied following the end of Roman rule in the early 5th century, to defend against pirate raids and the Anglo-Saxon invasions in the 6th century. Some hill forts were reoccupied by the Anglo-Saxons during the period of Viking raids in the 9th century.