A castle is a structure that is fortified for defence against an enemy and generally serves as a military headquarters dominating the surrounding countryside.

The mediæval castles of Cornwall are of motte and bailey type, and were introduced by the Normans. They consist of a large mound (or motte) topped by circular stone tower or keep, usually a replacement for an earlier wooden structure. The Motte is attached to a walled area (the bailey) where barracks blocks, workshops, stables and other domestic buildings could be securely sited.

Towards the end of the period, as Cornwall becomes militarily and strategically important to the defence of the realm, Henrician castles are built to defend important ports and harbours and provide early warning on the threat of invasion.