The site of Bolingbroke Castle includes significant architectural remains and associated earthworks surviving in very good condition. Being principally of one build, the structure is a valuable illustration of castle design and layout in the mid-13th century, and our understanding of it has been enhanced by recent partial excavation and consolidation. Associated earthworks of both medieval and post-medieval date have survived intact in the southern part of the site, preserving the relationships between a diversity of activities. Waterlogging in the area of the moat indicates the likely survival of organic remains. The castle is both well documented historically and well known as the birthplace of Henry IV; as a monument open to the public it thus functions as an important educational and recreational resource.
The monument includes the remains of Bolingbroke Castle, built in the early 13th century by Randulph de Blundevil, Earl of Chester and Lincoln. In the late 13th and early 14th centuries it served as an administrative centre for the extensive estates of Henry de Lacy, Earl of Lincoln, before passing by marriage in 1311 to the house of Lancaster. Henry IV, son of John of Gaunt and Blanche of Lancaster, was born here in 1366. The castle was extensively rebuilt in the 15th century, but by the early 17th century had fallen into disrepair. In October 1643 the Royalists stood siege here and were defeated by the Parliamentarians, who held the castle before leaving it to ruin. The structure further deteriorated through the succeeding centuries until the late 20th century when it was excavated and consolidated. The monument includes the remains of the castle and its associated earthworks.
Bolingbroke Castle is situated in a valley at the point where the wolds rise out of the fenland of eastern Lincolnshire. The site lies in an area of grassland on the south side of the village of Old Bolingbroke. In the northern part of the monument are the standing remains of the castle, a stone-built structure of roughly hexagonal form enclosing an area of approximately 0.25ha. The curtain wall, constructed of Spilsby sandstone with Ancaster limestone dressings, incorporates five round corner towers and a double-towered gatehouse. The wall stands to a height of up to 2m above the ground level of the interior, the towers up to 3m. The gatehouse, on the north, is composed of two round towers with a cobbled road between. Alterations to the eastern tower, including the insertion of brick walls and a staircase to a basement chamber, date from its later conversion to a prison. Later alterations, including limestone openings, are also evident in the castle's north western tower which continued in use into the 17th century as a store for the castle's records and accounts. The north eastern tower was levelled in post-medieval times and its position is marked by modern stone slabs.
The interior of the castle is largely level apart from the earthworks of structures which have been investigated by excavation. These include, adjoining the gatehouse and curtain wall on the north east, the remains of the castle's great hall, a rectangular structure with stone foundations constructed in the 15th century. Adjoining the north western tower are the earthworks of a porch and garderobe, with a kiln mound to the south. On the south side of the enclosure is a raised area, standing approximately 1m higher than the rest of the interior, representing the remains of a complex of service buildings.
The castle buildings are surrounded on all sides by a moat, partly water-filled, approximately 30m in width. It is crossed by two post-medieval causeways: that on the north leads from the gatehouse and replaces an earlier drawbridge, while that on the east runs across the levelled north east tower. The moat is surrounded on the south, west and south east by an external bank.
Outside the castle, in the southern part of the monument, is a large rectangular enclosure of approximately 4ha bounded on the north by the external bank of the castle moat, on the south and west by a further linear bank, and on the east by a series of interconnected ponds. The ponds are roughly rectangular in shape and form a chain running along the western edge of the stream, from which they are separated by a narrow bank. About 20m to the west, lying near the centre of the large enclosure and on the same alignment, is a rectangular earthwork approximately 90m x 60m consisting of a central area surrounded by a linear bank and external moat; there is a further external bank on the south and west. The banks stand to a height of up to 1m above the surrounding ground surface, and the moat 1m below. Channels have been cut through the northern and south western parts of this earthwork to connect it to drains running from the castle moat in the north towards the stream in the south. The enclosure in which this earthwork lies, together with the pond complex, are medieval in origin and represent an outer bailey of the castle for use as a tilting ground and animal enclosure with adjacent fishponds; these features are believed to have subsequently formed part of a garden. Documentary sources indicate that by the end of the 16th century the enclosure was in use as a 'Rout Yard', an animal pound for the collection of stray cattle. The central earthwork is thought to have been associated with the Civil War siege, later serving as a pen and watering place for empounded animals. Bolingbroke Castle is Listed Grade 1. (Scheduling Report)
Former castle, now ruin. Founded 1220-1230 by Randulph de Blundevill, Earl of Chester and Lincoln. Extended and altered C14 when it became part of the Duchy of Lancaster. Rebuilding C15, and C16 but dereliction had set in by 1600. In 1642 it was sleighted during the Civil War. Squared greenstone rubble, ashlar dressings. Courtyard plan with octagonal and round corner towers. Walls stand to about 6'0". The castle was the birth place of King Henry IV, son of John of Gaunt. It was excavated in the 1960s. (Listed Building Report)