In its final form, Basing House comprised two linked houses. The "Old House" replaced the keep of an older ringwork castle, so was located within a defensive ring of earthworks and walls, whilst the slightly later "New House" was located outside the defences. A bridge and gateway linked the two across and through the defences, a link that was to prove fatal in the final battle for the fortress. The "New House" 'annex' in itself had approximately 360 rooms and was built primarily to accommodate the multitude of people that accompanied royalty when they visited.