Hillfort building reached its peak at different times in different places. In most of Europe, including Britain, where there are more than 2,000 known hillfort sites, the great majority seem to have been built during the fifth and sixth centuries BC. Most of these were abandoned after around 400-350 BC, however, with only a very much smaller number continuing in use.
The development of hillforts is attributed to climatic deterioration and increasing population pressures leading to conflicts between neighbouring tribes. In Britain, it has also been associated with possible invasions or settlement by the peoples known as Celts. Hillforts of Iron-Age Britain had as much to do with expressions of status or prestige as with actual conflict. Some archaeologists have pointed out obvious defensive deficiencies in the structure of sites, many of which are only at their most impressive where they are most clearly visible, implying that their role as strongholds at times of war may be exaggerated. On the other hand, we know from the archaeological evidence, including finds such as slingshot, that some hillforts at least must have been the scenes of actual battles at some stage in their existence.
Whatever their individual functions and histories may have been, hardly any of them were in use by the later Iron Age. Some were reoccupied at the time of the Roman invasion, to provided centres of resistance to the conquest. Unfortunately for their occupants, not even of Maiden Castle could provide a defence against the might of the legions from Rome. The Romans occupied some forts, such as the military garrison at Hod Hill, and the temple at Brean Down, but others were destroyed and abandoned. Mass graves at Cadbury Castle indicate it was involved in the Boudiccan revolt in 60-61 AD. Many of the place names of these sites bear the suffix "-bury", meaning fort.
Some forts were reoccupied following the end of Roman rule, to defend against pirate raids, and the Anglo-Saxon invasions. The Wansdyke was a new linear earthwork connected to the existing hill fort at Maes Knoll, which defined the Celtic-Saxon border in south-west England during the period 577-652 AD. King Alfred established a network of coastal hill forts and lookout posts in Wessex, linked by a Herepath, or military road, which enabled his armies to cover Viking movements at sea.
After careful archaeological excavation, it has been found that many hill forts were just used to pen in cattle, horses, or other domesticated animals. Even those that were defensive settlements in the Iron Age, were sometimes used for corralling animals in later periods.