Scarborough Castle occupies a headland used for defense and settlement long before the medieval fortress. Archaeology has identified prehistoric activity and a Roman signal station, while a chapel later occupied the summit. William le Gros, earl of York, established a castle there during the civil war between Stephen and Matilda in the twelfth century. Its command of the harbor and North Sea coast made the site valuable for trade, naval observation, royal authority, and control of routes along northeastern England.

Henry II took the castle into royal hands and transformed it from the 1150s onward. The great keep, curtain walls, gates, halls, chambers, stores, and chapel created a major Crown fortress. King John and later monarchs invested in repairs and supplies during conflicts with barons and Scotland. Scarborough supported constables, soldiers, servants, prisoners, and officials even when kings were absent. The enclosed headland could shelter people and animals, while the keep displayed royal dominance over the town below.

Military use continued through the later Middle Ages, but artillery and coastal warfare demanded repeated adaptation. During the English Civil War, Sir Hugh Cholmley changed allegiance from Parliament to the king and held Scarborough for the Royalists. A parliamentary siege in 1645 brought prolonged bombardment, shortage, and damage before surrender. Royalists again occupied the castle in 1648 and were forced to capitulate after another siege. The fighting left the keep shattered and confirmed that the medieval fortress remained strategically useful but painfully vulnerable to cannon.

After the Civil War, the headland served as garrison, prison, and coast-defense position. Barracks and batteries were added or modified as threats changed. On 16 December 1914, German warships shelled Scarborough, striking the castle and town and killing civilians in the first major bombardment of Britain during the war. The attack gave the medieval ruins a new place in modern military history and exposed the North Sea coast to a form of warfare the old walls could not prevent.

Scarborough Castle is now conserved as a layered monument encompassing prehistoric remains, the Roman signal station, medieval keep and walls, Civil War damage, later batteries, and twentieth-century scars. Grassland management and archaeology protect evidence across the broad headland, not just the tallest masonry. The site's history demonstrates why commanding coastal places are repeatedly reused. Royal residence, prison, artillery platform, observation point, and public monument followed one another because the geography that first attracted settlement never ceased to matter.