The most impressive features of Helmsley castle are its large earthworks with two deep ditches that are cut down through the outcrop of rock on which the castle stands. This ringwork formed part of the original earth and timber castle built in the early 12th century by Walter Espec. Following his death in 1154 the castle passed to his brother-in-law, Peter de Roos, and it would be held by descendants of the de Roos line until 1688.

Robert de Roos rebuilt the castle in stone some time after 1186. He replaced the timber palisades with a stone curtain wall that had round corner towers and two entrances, one in the south-east below a strong square tower and one in the north in a gatehouse with paired round towers. He also added two great towers, the east tower and west tower. The defences were further strengthened in the mid 13th century with the addition of barbicans to the north and south gates. William de Roos inherited the castle in 1285 and made substantial changes to the property. He added another storey to the east tower, creating new accommodation, and he strengthened the south barbican. He added a new hall that abutted the west tower which he had converted into comfortable private apartments. The castle remained little changed until the 1560's when the west tower and hall were converted into a fine Tudor house for Edward Manners.

The castle defences remained untested until the Civil War, when, in 1644, a parliamentary force laid siege to the royalist held castle for three months. Lack of food finally forced the garrison to surrender and the castle defences were subsequently demolished to prevent any further threat. The east tower was blown up, and much of the debris still remains in the ditch below. The Tudor house was left intact, but was abandoned in the early 18th century in favour of a new house in the adjacent Duncombe Park.