The castle (also sometimes referred to as Gains Castle), was built in 1138, on the site of a former Iron Age fort. The site is likely to have been of strategic value, standing on high ground, having a well, and lying a short distance from the Roman road from Winchester to Sarum. Its original builder was the powerful Henry de Blois, a grandson of William I of England, and the younger brother of King Stephen. He was the Bishop of Winchester from 1129 until his death 1171, a prolific builder (including of Wolvesey Palace, the bishop's palace in Winchester), and one of the most powerful figures of his day. The castle was built during the Anarchy, a prolonged period of unrest and civil war between opposing factions led by King Stephen and the Empress Matilda. The Anarchy ended with the consensual accession of Matilda's son Henry II in 1154. Shortly afterwards, in 1155, Ashley Castle was slighted, along with other castles built by Henry of Blois. Although Henry II was the son of the Empress Matilda, and Henry of Blois was the brother of her adversary, King Stephen, this was not necessarily an act of retaliation against Henry of Blois; in re-asserting royal authority through the realm, Henry II destroyed numerous unauthorised castles that had been raised during the Anarchy.
However, Ashley castle was to have a second lease of life. Within fifty years, during the reign of Henry II's son King John, Ashley Castle was restored by a new owner, William Briwere the elder. William Briwere held the manor of Ashley, was a leading counsellor of King John (and later of his son King Henry III), and indeed in early accounts was referred to as one of his "evil advisers". He was also a signatory of Magna Carta (though seemingly a reluctant one), and founded nearby Mottisfont Abbey, a house of Augustinian canons. In 1200 King John granted Briwere a licence to crenellate a number of castles, including, in Hampshire, one at either Stockbridge or Ashley (at Briwere's election). Briwere chose Ashley (referred to in the grant as "Esleg"). In an age when the crown saw a potential rebel stronghold in each castle held by the nobility, and the Anarchy was a recent memory, permission to fortify in this way was a rare privilege. King John is documented as having stayed frequently at Ashley, possibly in connection with hunting expeditions in the royal Forest of Bere - of which the nearby river Test formed the western boundary at the time. Official crown business is known to have been transacted from the castle, as a number of royal letters from this era are headed 'Ashley'. Ashley was the administrative headquarters of West Bere, which stretched from the river Itchen to the river Test. The warden had his manor there, and would have stayed in the castle when in residence (Briwere also had other estates elsewhere). Many English forests had a castle headquarters of this sort. In addition to serving as a residence, pleas were heard, poachers sometimes imprisoned and forest officers housed there. Despite its role as an administrative base for crown land, Ashley Castle appears always to have been considered as part of the manor of Ashley rather than as crown property, and the interests of the wardens were often allowed to overrule the interest of the sovereigns whose property they were supposed to be protecting.
In its earliest form, the castle included a circular area enclosed by a bank and deep ditch and containing a wooden tower. Extending in a loop from the first bank was another, similar bank enclosing a court - the outer bailey - which contained domestic buildings and the parish church (which probably predates the castle). Both banks were surmounted by wooden palisades.
- Philip Davis. "Gains Castle, Ashley". gatehouse-gazetteer.info.
- "Recent Excavations in the Keep of Farnham Castle, Surrey" (PDF).
- "History of Ashley Castle".
- "Parishes: Ashley". british-history.ac.uk.
- Wendover (iii. 238)
- Brewer, William (DNB00)|Hunt, William, biography of Brewer, Briwere, or Bruer, William (d. 1226), published in Dictionary of National Biography, Volume 6, 1885–1900
- "The Gatehouse website record of a licence to crenellate for Gains Castle, Ashley granted on 1200 June 6". gatehouse-gazetteer.info.
- Cal. Rot. Chart. (Rec. Com.), i, 70.
- King John's Itin. Rot. Lit. Pat. (Rec. Com.), I
- 'The History of Ashley, Kate Gilbert, published by Hampshire County Council, 1992, ISBN 1 873595 25 5
- Williams-Freeman (1913)
- Historic England. "Details from listed building database (1013635)". National Heritage List for England. Retrieved 27 September 2015.
- Fry, Plantagenet Somerset (1980). The David & Charles Book of Castles. Newton Abbot: David & Charles. p. 180. ISBN 0-7153-7976-3.