The modern state originated in medieval and early modern Europe, spread to all parts of the world, and quickly became the dominant form of political organization. Quantitative scholarship on the historical development of the modern state has lacked a measure of historical statehood that reflects the defining feature of the modern state: the monopoly on the use of physical force within a territory. We propose a new measure of statehood, the share of castles controlled by the crown; one that starts from
the canonical definition and allows us to trace the origins of the modern state in the period when it actually happened. The measure is based on an original data set on castle ownership across medieval and early modern Europe. In this paper, we introduce the Castles data. To test the validity of the proposed measure, we show that the measure correlates well with
historiographical narratives of state formation in Denmark, Sweden, and England.