Decision making under constraints is the provenance of economics, and the decisions of medieval rulers regarding war form a fertile area for the study of the making of choices. Potentially, there is a marvelous case for examining whether military history is amenable to economic analysis. The pithy popular image of the High Middle Ages—castles and knights—is actually helpful. Studying the economics of castles and warfare can also tell us a lot about the time. This chapter examines the concept of opportunity cost with respect to castle-based warfare during the High Middle Ages. It presents an accounting for the sheer number of castles, especially in relation to English kings. It then assesses the cost of castle building and show why, despite the truly enormous cost, the advantages outweighed the cost. Armies were not irrelevant but their cost was high and the advantages slight. The chapter concludes by pointing to where economic principles other than the principle of opportunity cost may be seen in the history of castle building.