Herstmonceux Castle, with attached bridges to north and south and causeway with moat retaining walls to west.

Herstmonceux Castle, with attached bridges to north and south and causeway with moat retaining walls to west.

Castle and country house, built circa 1441 when a licence to crenellate was granted to Sir Roger Fiennes. The building was further embellished in the mid-16th century for Baroness and Lord Dacre, and altered in the mid to late 17th century for Lord Dacre. Part of the castle was demolished between 1776 and 1777 for Robert Hare. The building was restored and rebuilt in the early 20th century, mostly between 1911 and 1912 for Lieutenant Colonel Claude Lowther, and again in the 1930s for Sir Paul Latham.

The castle is constructed of red brick in English bond with some blue header diaper work, with stone dressings and plain tile roofs. The building is square on plan with an inner courtyard, originally divided into four courts and containing a Great Hall. These internal features and the internal walls were demolished in the 18th century. The south range and the south ends of the east and west ranges were restored by Lowther, while the remainder was restored by Latham.

The castle stands two storeys with attic and basement in parts. It is five bays wide and four bays deep, featuring tapering polygonal towers at the corners and between bays, taller at the angles and centre. The exterior is built and restored in 15th-century style with one-light or two-light windows, some transomed. The courtyard side displays wider windows, some with cusped or round-headed lights. Doorways feature four-centred-arched or segmental-arched moulded or chamfered openings with 20th-century studded board doors. A tall plinth with moulded offset runs around the building, with a moulded string below an embattled parapet with roll-moulded coping. Rainwater pipes have decorative initialled heads. Stacks feature ribbed and corniced clustered flues. Roofs are steeply pitched with roll-moulded coping, some with hipped ends.

The south (entrance) elevation is dominated by a three-storey central gate tower containing a tall recess with a wide panelled door and a window of two cusped, transomed lights above. Grooves for former drawbridge arms are visible. On the second floor, two transomed windows of two round-headed lights flank a coat of arms of Sir Roger Fiennes. Flanking towers have gun ports at the base, looped arrow slits, and machicolated parapets with arrow slits to the merlons, with towers rising above as drums. A long bridge of eight arches projects from the gate tower, mostly of 20th-century date. The central arch is wider and shallower than the others. The bridge features cutwaters, a stone parapet, and a central corbelled embrasure with flanking tower buttresses.

On the north side, the central gate towers formerly contained rooms on the lower floors, of which truncated walls and a first-floor fireplace fragment remain. A machicolated parapet runs along this elevation. At the left end of the range are 17th-century window openings with later eighteen-pane sashes.

The west side features an attached causeway containing a basement room. A three half-arched bridge on the south side of the causeway projects into the moat, with walling returning as moat retaining walls. The main range has a basement doorway with side-lights in a chamfered embrasure.

The east side displays a 16th-century first-floor bow window to the second tower. Tall windows to the central tower, which contains a chapel, are notable. The right half of the range has older windows blocked and replaced with larger 17th-century openings with later eighteen-pane sashes.

The courtyard contains a seven-bay arcade to the north side and a central corbelled stack with a clock. A three-bay Great Hall of the 1930s, now a library, occupies the west side with decorative tracery to its windows and an offset buttress. The gable of the former chapel on the east side features perpendicular tracery to a window, a two-storey bay window, and two crow-stepped gabled attic windows to its left. Several doorways and a two-storey bay window face the south side. Hipped-roofed dormers are distributed throughout. A brick-lined well sits in the south-west corner.

The interior retains some original features, including fireplaces, privies, doorways, a dungeon, and a brick-lined dovecote in the south-east tower. Many fine features were brought into the castle from elsewhere during early 20th-century restoration, including doors, fireplaces, and panelling.

In the south range, the porter's room has an old fireplace and a relocated linenfold door found in the cellar. A reused traceried wooden panelling is incorporated in the rebuilt dining room fireplace. The stair hall features a fine early 17th-century wooden stair brought from Theobalds in Hertfordshire, with strapwork roundels between square vase balusters, elaborate relief decoration, and lion finials holding shields. An elaborate doorcase of the same period stands at the head of the stair, accompanied by a ribbed ceiling with pendant finials.

The Drummers Room contains reused panelling dated 1697, with fluted pilasters and a frieze. An elaborately arcaded and fluted-pilastered overmantel decorates this space. The Green Room on the second floor has a restored fireplace with crests and beasts on its hood, moulded beams and bosses, and a reused traceried panel below the courtyard window.

In the north range, a very fine late 17th-century stair brought from Wheatley Hall in Doncaster, possibly from the workshop of Grinling Gibbons, features baskets-of-flowers and pendant finials to newels. The balustrades display open, leafy scrollwork with flower roundels. At the head of the stair are two elaborately carved doorcases in a similar style with shields in broken pediments. The former ball room has an arched ceiling with decorative plasterwork and 17th-century-style panelling. A reused elaborately-decorated 17th-century wooden fireplace overmantel from Madingley Hall in Cambridgeshire features two orders of caryatids and embossed panels.

In the east range, the former chapel contains a reused 15th-century wooden screen from France set in the west wall. The former drawing room has an elaborate stone fireplace in 1930s 16th-century style. In the ante room is a reused richly decorated fireplace with griffins and portrait roundels.

The 15th-century castle was well restored in the early 20th century. The many fine features brought in during this restoration period add significantly to its architectural importance.