The medieval castle is one of the most popular topics in medieval historiography and interest in this structure has institutionalized it in modern medieval scholarship. Unfortunately, this does not mean that modern historians understand it. The problem lies in the narrow and isolationist definition used by many scholars who see it as simply a fortified private residence representing and defending power. This thesis will demonstrate that the castle’s contemporaries understood it as an identifiable and distinguishable structure and symbol with a singular yet multi-dimensional characteristics as a fortified, personal, and multifunctional resource. The twelfth-century Anglo-Norman realm has been chosen as a focus for this thesis because of the specific differences between the reigns of Henry I, Stephen, and Henry II. This period, particularly the nineteen years of Stephen’s reign, experienced significant castle warfare, which provides a great deal of material for this study. In chapters 1-3, each of the above characteristics and their corresponding details will be analysed individually. In chapter 4, three case studies will be presented to demonstrate how these independent characteristics were perceived of as acting simultaneously. Chapter 5 will compare perceptions of castles to other medieval buildings. Finally, chapter 6 will test the definition’s legitimacy by applying it beyond the twelfth-century Anglo-Norman realm. This dissertation will demonstrate that there was a contemporary understanding of the castle which encompassed its fortified nature, its personal possession, and its multifunctional resourcefulness. If we are to understand this phenomenon as its contemporaries did, then we need to alter our modern definition and expand our understanding in order to come to a truer and more complete appreciation of this essential resource in the Middle Ages.