The ruins of Odiham Castle in Hampshire consist of the tall walls from an octagonal, three story keep, built by King John in the 1220s, the castle itself having been established several years earlier, in 1207. There is now no trace of the other components, which included a kitchen, chapel and great hall, though the moat which surrounds the site does survive, as a low, waterlogged ditch. Despite being only half a mile from the M4, and near several sizeable towns, the castle still has a peaceful, secluded setting, surrounded by trees, on the inside of a bend along the Whitewater River, used in medieval times to supply the moat. The castle also adjoins the Basingstoke Canal, constructed in 1792, and cutting through part of the outer bailey, the position of which is still discernible, as an overgrown earthwork to the southeast of the inner bailey, centred on the keep.
Windows, a fireplace and a doorway are still evident in the keep, and although the walls have lost all of their original decorative cladding and now comprise only the rough flint core, the ruin is still impressive, rising nearly to the full height in places (30 feet), and mostly complete in circumference, missing just one significant section of wall, to the west. The site is reached by a short walk along the canal towpath, starting from a minor road (Tunnel Lane/Mill Lane) in North Warnborough, one mile north of Odiham. The fortification is also known as King John's Castle, and is one of three built by the king, the others being in Ireland, at Carlingford and Limerick.
The location of Odiham Castle, in the countryside along a minor river, not close to any settlements of importance, was chosen for being half way (25 miles) between the royal residences at Winchester and Windsor, and so a logical place to break the two-day journey; the primary purpose was residential rather than defensive. The king's journey between these places can be followed by the Three Castles Path, a 60 mile route across the countryside. John is believed to have first visited the Odiham site in 1204, and construction of the castle began three years later, on land granted by the local nobleman Robert de Parker; the initial layout consisted of a two floor keep within a rectangular enclosure, protected by a moat, fence and embankment. The castle was briefly captured and damaged in 1216 by French forces but soon reclaimed, and the defences strengthened by replacing the keep with a thicker-walled structure of three floors, with an octagonal cross-section; an unusual design, representing an intermediate stage in the development of keeps from square to round. Each of the eight angles was supported with a narrow, multi-stage buttress. The castle passed by marriage to Simon de Montfort in 1238, and reverted to the Crown in 1265, after which it was occasionally used as a prison, and later (in the 15th century) as a royal hunting lodge, but had become ruinous by the start of the 17th century, when most of the facing stones had been removed for use elsewhere. Limited restoration was carried out in 2007.
Parking is available in a layby along Tunnel Lane, shortly before the road crosses the canal on a swing bridge. The towpath runs along the north side of the canal, through a corridor of trees, and reaches the castle after a quarter of a mile, surrounded by more extensive woodland. Another path circles the castle and enters through the original doorway in the east wall, now merged with the window above to create a larger opening. At the centre is a recently-installed monument to the Magna Carta, as it is believed that King John was resident here at Odiham immediately prior to the June 15th 1215 meeting with the Barons, at Runnymede. Railings encircle the central viewing area, while a fence runs around the exterior, so the walls cannot be approached close-up, but all details can clearly be seen. The keep now is split into two fragments, one and parts of another two of the original eight walls to the north, and four to the south. Square joist holes show where floor supports once rested, meeting at a central timber column that spanned the full height of the structure. The walls contain six complete arched window openings, a fireplace and a few smaller niches, while at the top are several pinnacles, once forming the side of the smaller windows along the upper story.