Sandown Castle was built as a consequence of international tensions between England, France and the Holy Roman Empire in the final years of the reign of King Henry VIII.

Traditionally the Crown had left coastal defences to the local lords and communities, only taking a small role in building and maintaining fortifications, and while France and the Empire remained in conflict with one another, maritime raids were common but an actual invasion of England seemed unlikely.

[1] Modest defences, based around simple blockhouses and towers, existed in the south-west and along the Sussex coast, with a few more impressive works in the north of England, but in general the fortifications were very limited in scale.

[11] The angular bastions echoed contemporary Italian thinking on military architecture, and may have been influenced by Richard Lee, the King's Surveyor of Works, as well as by Portinari's own continental background.

Admiral Claude d'Annebault crossed the Channel and arrived off the Solent with 200 ships on 19 July, where the local authorities feared Sandown Castle might be the target of a night attack.