St Andrew's 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 modest 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] Basic 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.

[6] In response, Henry issued an order, called a "device", in 1539, giving instructions for the "defence of the realm in time of invasion" and the construction of forts along the English coastline.

[11] William Paulet, later the Marquess of Winchester, was appointed as Keeper and Captain of the castle in 1547, being paid £19 each year to perform the role; his garrison included a master gunner, a porter and six soldiers.