We present the design and evaluation of the Augmented Climbing Wall (ACW). The system combines computer vision and interactive projected graphics for motivating and instructing indoor wall climbing. We have installed the system in a commercial climbing center, where it has been successfully used by hundreds of climbers, including both children and adults. Our primary contribution is a novel movement-based game system that can inform the design of future games and augmented sports. We evaluate ACW based on three user studies (N=50, N=10, N=10) and further observations and interviews. We highlight three central themes of how digital augmentation can contribute to a sport: increasing diversity of movement and challenges, enabling user-created content in an otherwise risky environment, and enabling procedurally generated content. We further discuss how ACW represents an underexplored class of interactive systems, i.e., proximity interaction on wall-sized interactive surfaces, which presents novel human-computer interaction challenges.