Castles are defined as buildings with two primary purposes: places of defence and attack, and places in which to live. And they belonged to a particular individual, unlike prehistoric forts, for example, which were communal in their purpose and function.
The idea of castles grew out of the fortifications that men had used for centuries. In England, some of the most conspicuous of these are the great hill forts built in the Iron Age. Outstanding examples of these are Maiden Castle and Hod Hill in Dorset, and Danebury in Hampshire. These monuments consist of enormous earthen ramparts, originally topped by wooden walls and walkways, and surrounded by deep ditches. Some of these were only used at certain times of the year, or during periods of war, but at Danebury, for example, excavations have revealed the remains of hundreds of huts, as well as other features, including thousands of pits used for storing grain. Such forts were at the core of highly organized and sophisticated tribal societies.
It was the Roman invasion that sounded the death-knell for hill forts. The Romans overran them with comparative ease despite their defences, and the kind of society the Romans introduced revolved around towns. Almost all the ancient hill forts were abandoned. An exception is Old Sarum, the precursor of Salisbury in Wiltshire. Here the hill fort was re-used several times and abandoned only in the 13th century.
Roman military fortifications were built with typical efficiency. Even temporary marching camps were well built, with many still surviving. Such camps could be built at the end of each day’s advance, as a base for future advances, or as a place to fall back to.
After the Romans left, some of the old hill forts were re-occupied, including Cadbury in Somerset, which some think may have been the headquarters of King Arthur. A systematic fortification-building programme was undertaken in the 9th century by King Alfred to counter attacks by Vikings. Called burhs, these forts were highly effective when manned by determined troops.
The Normans introduced the fortifications that we know as castles. They based these on structures in use on the European mainland since at least the 9th century. Within a short time of the invasion, Norman castles were built all over the British Isles; they were to become the symbol of Norman might and conquest, and were the focus of great historical events for the next 700 years.
Dun Aenghus, Ireland: promontory forts like this exist in large numbers along the coast of Ireland, and there are some on the British coast. Typically built of drystone walling, they could easily be defended, but there was no easy escape for the defenders if their attackers broke through.