This thesis addresses a neglected area of castles studies - the spiral stair. It studies the origins, evolution, placing, structure, role, significance and meaning of spiral stairs in medieval stone castles between 1066 and 1500, so covering the rise, zenith and decline of the castle in England and Wales. Although focussed upon England and Wales, it has a wider geographical spread across Ireland, Scotland, Europe, the Middle East and Japan with particular regard to castles and on even wider when searching for the origins of the spiral stair, encompassing the whole globe. The date range was also extended, both much earlier than 1066 when searching for these origins and very selectively beyond 1500 when exploring how the spiral was used in the later medieval and early modern periods. It is proposed that the first known spiral stair was employed in Trajan's Column in the first century AD, that it was then used more selectively in secular and later ecclesiastical buildings during the first millennium AD and that, from the eleventh century onwards, the spiral stair became a common feature of the medieval castle. From the emergence of the spiral stair in Rome, this thesis places its principal use in European elite and ecclesiastical structures. Focusing on the castle, this thesis argues that it was employed as a vertical boundary marker to signal and control movement between two different types of spaces, from a more public to a more private space and from a general or less restricted space to a space which was more restricted, often elite domestic quarters. This use of the spiral is seen in and is traced through different types of English and Welsh castles, from stronghold to enclosure and on to the so-called sham or cult castles of the late medieval period. The thesis also looks at the spiral in a range of medieval castles and other defensive buildings outside England and Wales and finds that, in the main, spirals were employed in the same way. It also explores the presence and role of the spiral within other medieval buildings, both in England and Wales and further afield, and argues that, although there are some exceptions and variations, in the main spiral stairs played the same role in those buildings. This thesis interprets the spiral stair within the medieval castle as a key component of the landscape of lordship and argues that the interpretation of this elite landscape, hitherto focused on the environs and outward appearance of the castle, should not stop at the castle gate but should move inside. Accordingly, this thesis takes a step to bring the interior of the castle deeper into research and discussion; to explore individual items and features within the castle; and to consider their placing, access and meaning within the medieval world.