The term Iron Age refers to the period in a civilisation's development at which time iron working was the most sophisticated form of metalworking achieved. Though well made bronze tools far surpass iron tools as far as hardness and utility are concerned the abundance of iron ore sources made iron cheap and contributed greatly to its adoption as the most common metallurgical process. The Iron Age is part of the Three-age system for prehistoric societies.

• Earlier Bronze Age
• Later Middle Ages and in parts of Europe, Roman occupation. In Sweden and partly in other nordic countries, the last part of the Iron Age (about 800 - 1100) is commonly called the Viking Age.

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.

Structures:

• Broch
• Crannog
• Dun
• Fort
• Roundhouse
• Wag
• Wheelhouse