CASTLES WERE IMPORTANT STRUCTURES in late medieval England, serving a variety of functions, such as residences, prisons and administrative centres. They had also played a significant role in English warfare ever since their introduction by the Normans in the eleventh century. As fortified buildings, they could be used by defenders to protect a specific location from attack, or alternatively they could play an offensive role as a secure base from which a garrison could exert control over the surrounding area. The security provided by castles also meant that they could be employed as arsenals and as supply depots for field armies. This can be seen with the castles constructed at great expense by Edward I in the late thirteenth century to secure his conquest of North Wales. These castles are large and impressive masonry structures, often with multiple circuits of walls, strategically sited near the coast or rivers. Later English success in crushing the rebellions led by Madog ap Llywelyn in 1294–5 and Owain Glyndŵr in the early fifteenth century was due, at least in part, to the possession of these castles, which could be easily supplied by sea, defended by small garrisons and used as forward bases to take the war to the Welsh rebels. However, it was the final major programme of castle construction initiated by an English monarch. This was due, at least in part, to the significant increase in the number of castles in the possession of the crown in the late Middle Ages, notably those acquired from the estates of the duchies of Lancaster and York by the accessions to the throne of Henry IV and Edward IV in the fifteenth century.

The last royal castle constructed in the Middle Ages was Queenborough, situated on the Isle of Sheppey in Kent. This was built by order of Edward III between 1366 and 1377, with its concentric design strongly indicating that it was designed to provide coastal defence, further evidence of which can be seen by the provision of artillery for its safeguard. By the late Middle Ages, however, most English castles were very rarely, if ever, involved in military operations, except for those in frontier regions such as near the Scottish border. As a result of this, the vast majority of castles, even important royal ones, were only garrisoned during times of crisis, notably in 1385–6.