In Britain, the Iron Age lasted from about the 5th century BC (although some believe it to have started significantly later, around 1st century BC) to the 4th century AD. Defensive structures dating from this time are often impressive (for example, the brochs in Scotland). This is possibly because of greater tension between better structured groups, although there are suggestions that in the latter phases of the Iron Age they existed simply to indicate wealth. Either way, during periods of Roman occupation the evidence suggests the defensive structures served their purpose well. Many were re-used by later cultures, such as the Picts, in the early Medieval period.