At the time of the Norman Conquest, William the Conqueror granted overlordship of the Isle of Wight to his relative William FitzOsbern who began the construction of a castle at Carisbrooke. The lordship of the Island passed to the De Redvers family in 1101, with the hereditary rights and privileges that accompanied it, until the Countess Isabella De Fortibus, the last survivor in the family, sold the Island to Edward 1 in 1293 for six thousand marks.

The acquisition of full control of the Island by the crown was important because the Island's vulnerability to invasion. The lordship of the Island was now a royal appointment. One of the lords of the Island - Henry Beauchamp, Duke of Warwick - was actually given a title of King to the Isle of Wight in 1444 by Henry V1, who attended the ceremony and placed the crown on his head.

During the Hundred Years' War, the Island, like much of the south coast, became a target for marauding French. The Island's only fortification, the Norman Castle at Carisbrooke, assumed a defence role for which it was unsuited owing to its central position; The French could land on the coast and burn and plunder while ignoring the castle. Towns and villages like Yarmouth, Newtown and Newport were sometimes attacked and burned. It is said that in 1377 a party of French fell into an ambush on the outskirts of Newport and were cut to pieces in Dead Man's Lane, now Trafalgar Road. They are supposed to be buried at 'Noddies Hill', now known as Nodehill or Upper St. James' Street. On the same occasion the French besieged the castle, but, according to the legend, retired on the death of their commander, shot from the castle's west wall. Concern about the French attacks is shown in the frequent modifications made to the Castle's defences in the 14th Century.

The strategic importance of the Island increased with the development of Portsmouth as a permanent naval base. Henry VIII accordingly built additional fortifications on the island at Yarmouth, East & West Cowes and Sandown, sometimes re-using stone from dissolved monasteries as building material. Sir Richard Worsley, Captain of the Island at this time, successfully commanded the resistance to the last of the French attacks in 1545, at the time of the loss of the Mary Rose.

During the reign of Elizabeth 1, the Island was again threatened by invasion, this time from Spain. Sir George Carey, cousin of the Queen and Captain of the Island, took up residence at Carisbrooke Castle in 1583 and undertook repairs to the defences. Although the Armada was defeated in 1588, the Spanish threat remained, and the outer fortifications of Carisbrooke Castle were built between 1597 and 1602 in response to the invasion scare.