Device Forts The Device Forts , also known as Henrician Castles , are a series of artillery fortification s built to defend the southern coast of England by Henry VIII . After his divorce of Catherine of Aragon England was left politically isolated , and the Peace of Nice between France and Spain in 1538 aroused fears of invasion . This threat stimulated the beginning of the first phase of the largest defence programme since Saxon times . This first phase was known as the 1539 device programme which was followed after renewed threats from the French by a second programme , the 1544 device programme . Henry ’ s coastal defences ranged from earthen bulwarks to small blockhouses and artillery towers to state of the art Italianate style fortifications . Henry took a personal interest in the military engineer ing techniques of the time , and approved and amended the designs himself . Although they were built to defend England during Henry ’ s reign , many of them were used in the English Civil War and were refortified at various times during the Napoleonic wars , First and Second World Wars . 1539 Device programme . Before building coastal defences around southern England , Henry VIII built fortifications in the north of the country , although much of his work no longer survives . Most of the work was making additions to existing castles to modernise them , as was done at Carlisle and Berwick - on - Tweed . In 1534 the Act of Supremacy passed by the Parliament of England made Henry VIII the head of the English church , challenging the authority of the papacy . Pope Paul III negotiated a peace in 1538 between Charles V , the Holy Roman Emperor , and King Francis I of France . The peace threatened England , and in response Henry VIII undertook a programme of building forts along England ' s south coast . In the first phase of Henry ’ s fortification programme 30 castles or forts were built . A curvilinear form of fortification was developed which consisted of squat artillery towers surrounded by round bastions to provide platforms for artillery . Good examples of these are Walmer Castle and Deal Castle in Kent , Calshot Castle and Hurst Castle in Hampshire and St Mawes Castle in Cornwall . The Castles of the Downs consisted of Deal Castle , Sandown Castle , Kent and Walmer Castle , which created a defensive line to protect the good landing grounds and strategic anchorage between the Goodwin Sands and the coast ( known as the Downs ) . They were linked by earthworks and bulwarks to provide further defence along this important stretch of coastline . 1544 Device programme . This was begun after renewed threat from the French in the 1540s and concentrated on the vulnerable area around the Solent estuary and the Southampton and Portsmouth ports . This later programme of coastal defences reflect new developments in fortifications , especially in the introduction of Italianate designs of the use of angular features . The earlier curvilinear designs of castles were replaced by square keeps surrounded by angular or arrow - head shaped bastions . Good examples of these are Yarmouth Castle and Southsea Castle built around 1545 . In 1547 – 8 , an inventory was made of crown possessions and this details the cannon and hand - arms of each fort . List of Henrician castles . Some of the key forts or castles are : Design . Military historian D . J . Cathcart King contrasts Device Forts with medieval fortifications , noting that " by medieval standards their profile is squat ; their battlements , unpierced by loops , have rounded merlons and widely - splayed embrasures with sloping sills , to reduce damage by gunfire ; and they are robustly built . " The early artillery forts are generally in the form of a central round tower surrounded by a variety of concentric elements . Short and squat , with normally 3 tiers of long - distance offensive armament and a couple of tiers of defensive armament . The bays had wide splays for easy traverse of the guns , walls were thick and curved to deflect shot , and the medieval portcullis es , murderhole s and drawbridge s were perpetuated . The later forts reflected new developments in fortifications , especially in the introduction of Italianate designs of the use of angular features . They had square keeps surrounded by angular or arrow - head shaped bastions , such as at Yarmouth .