Grimsbury Castle


Heritage Category:
Scheduled Monument
List Entry Number:
Date first listed:
Date of most recent amendment:
Location Description:
Grimsbury Castle hillfort lies about 0.8km to the south south-east of Hermitage in West Berkshire


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The building or site itself may lie within the boundary of more than one authority.

Location Description:
Grimsbury Castle hillfort lies about 0.8km to the south south-east of Hermitage in West Berkshire
West Berkshire (Unitary Authority)
National Grid Reference:


An Iron Age slight univallate hillfort.

Reasons for Designation

The slight univallate hillfort known as Grimsbury Castle is scheduled for the following principal reasons: * Survival: the hillfort survives well with earthworks to over 2m high; * Potential: only a small part of the hillfort has been excavated so that there is extensive archaeological potential remaining for future investigation. This hillfort also has the potential to help understand the transition between Bronze Age and Iron Age communities; * Rarity: slight univallate hillforts are rare, there being only about 150 examples surviving nationally.


Grimsbury Castle is an Iron Age hillfort. Iron Age hillforts are often large striking monuments with monumental defensive earthworks. They were built and occupied during the period from about 900 to 100BC, though many were re-used. There are more than 3,000 hillforts in the British Isles, although they are not distributed equally throughout the country; the main concentrations being found in Wessex, the Welsh marshes and the south-east. Whether they were primarily defended sites or had a varied range of purposes in Iron Age society is the subject of current debate; and because there is still much to learn about them the archaeological potential of hillforts is very great. Some hillforts acquired a new defensive role in the C20 when they became the location for military installations, and indeed there are a number of Second World War two-man slit trenches in the western rampart of Grimsbury Castle.

Although many hillforts are large, some are quite small and Grimsbury Castle falls into this category. The type of hillfort is also classified by whether it has a single defensive bank and ditch or multiple defences. Grimsbury Castle is a slight univallate hillfort having primarily one bank and ditch. Univallate hillforts are the earliest hillfort type chronologically. The shapes of hillforts also tend to conform to the contours of the ground they occupy. Some, including Grimsbury Castle, are built on spurs and are called ‘promontory forts’.

Slight univallate hillforts are defined as enclosures of various shapes, generally between 1ha and 10ha in size. They are situated on or close to hilltops and defined by a single line of earthworks. They date to between the Late Bronze Age and the Early Iron Age (eight to the fifth centuries BC), the majority being used for 150 to 200 years prior to their abandonment or reconstruction. Slight univallate hillforts have variously been interpreted as stock enclosures, redistribution centres, places of refuge and permanent settlements. The earthworks generally include a rampart, narrow level berm, external ditch and counterscarp bank, while access to the interior is usually provided by two entrances comprising either simple gaps in the earthwork or inturned ramparts. Excavation indicates the occasional presence of portal gateways, while more elaborate features such as overlapping ramparts and outworks are limited to only a few examples. Internal features included timber or stone round houses; large storage pits and hearths; scattered postholes, stakeholes and gullies; and square or rectangular buildings supported by four to six posts, often represented by postholes , and interpreted as raised granaries. Slight univallate hillforts are rare with around 150 examples recorded natonally. Although on a national scale the number is low, in Devon they comprise one of the major classes of hillfort. In other areas where the distribution is relatively dense, for example, Wessex, Sussex, the Cotswolds and the Chilterns, hillforts belonging to a number of different classes occur within the same region. Examples are also recorded in eastern England, the Welsh Marshes and central and southern England.

Grimsbury Castle hillfort was partially excavated by P Wood in 1958, but nothing of importance was found. Further excavations in 1960 were confined to the inner, north, side of the western entrance. It was shown that this simple entrance was possibly reinforced by a timber palisade and was probably of two periods of construction; it was subsequently strengthened by a flint wall. Very few finds were made, most of them including ‘pot boilers’, sling pebbles and worked flints, were of little use in dating the hillfort. From the ditch fill only three small sherds of Iron Age pottery were recovered which were dated to the middle or late part of the Early Iron Age and a portion of a Greensand quern stone was also found.

It is recorded, in a note in the 1860 Journal of the British Archaeological Association, that there are six or seven mounds 20ft long by 12ft wide inside the earthwork. These were excavated c1860 but nothing found; they were thought by field investigators in 1963 to be pillow mounds (artificial warrens used to provide a continuous supply of rabbit meat). Some possible low mounds were identified during the site inspection, but not thought to be pillow mounds. The hillfort appears to have been utilised during the Second World War since the remains of a number of back-filled two-man slit trenches, each about 2m long by about 0.8m wide are present in the western bank of the hillfort. It is not known if they were dug as part of a defence scheme, by the Home Guard, or simply as training exercises for troops in the area. The report of a third western (outer) bank and ditch documented by R Greenaway in 1979 was investigated by English Heritage (2014) but not found.


Grimsbury Castle is a slight univallate hillfort and also a promontory fort of 3.2 hectares (8 acres) almost entirely covered in trees. It has an inner bank, outer ditch and counterscarp bank as well as an outwork, comprising a bank, to the west of the hillfort. The hillfort is roughly triangular in plan, generally following the contours of the hill. The apices of the triangle are to the north, south-west and south-east. Height from bottom of ditch to top of bank is generally 2m or more.

The western outwork curves in an arc protecting the western side of the hillfort at a distance of between 40m and 60m from the counterscarp bank of the hillfort.

ENTRANCES There are two obvious entrances, both of which have associated outworks; the most elaborate is the western entrance where the hillfort’s bank and ditch are inturned slightly leaving an access of about 11m at its narrowest point. Here the inner bank is 2.5m high and the counterscarp bank to about 1m high. About 30m to the west of this entrance is the remains of an outer entrance work, part of the western outwork, comprising a complex of banks and a banked ditch about 2.5m deep. Such outworks are reasonably rare. This outer entrance work adjoins the western outwork bank which stands to about 1m high. The entrance in the northern apex of the hillfort triangle is about 11m wide and accommodates a modern road which follows the line of the original holloway rising uphill from the north. This original holloway can be seen as a 2m high double bank and ditch following the line of the modern road. It becomes a double ditch as it approaches the hillfort.

OUTWORK The outwork to the west is thought to be of two periods of construction, the northern, and apparently earlier, part appears as a slight break in slope to 1m high and is thought to possibly be associated with enclosing stock, whilst the southern section is a substantial bank to 1.5m high and clearly of a defensive nature.

EXTENT OF SCHEDULING The scheduling aims to protect the full known extent of the hillfort including its interior, its banks and ditches and the western outwork. The maximum extent of the monument is about 547m NE-SW by about 400m NW-SE. A 5m margin is included for the support and protection of the monument.

A number of features are excluded from the scheduling; these include all roads and their make-up, The Folly (Lodge) and its outbuildings and all signs and notice boards, although the ground beneath them is included.


The contents of this record have been generated from a legacy data system.

Legacy System number:
WB 105
Legacy System:


Books and journals
'Journal of the british Archaeological Association' in 15th Sept - Grimsbury, , Vol. 16, (1860), 229-30
Wood, P, 'berkshire Archaeological Journal' in The Early Iron Age Camp Called Grimsbury Castle, Near Hermitage, (1959)


This monument is scheduled under the Ancient Monuments and Archaeological Areas Act 1979 as amended as it appears to the Secretary of State to be of national importance. This entry is a copy, the original is held by the Department for Digital, Culture, Media and Sport.

End of official listing

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