This small backyard structure allows the homeowners to practice rock climbing, their favorite sport. It has two parts: a taller cubic form contains the climbing gym itself, and a lower rectangular volume that wraps around the gym and houses circulation, a powder room, and a single parking space. To maximize the climbing wall height while complying with zoning restrictions, the floor of the gym is set 30” below grade. Oversized aluminum windows face the main house; a wood trellis filters the afternoon sun. Interior finishes include a polished concrete floor and plywood panels in the gym; on the outside, redwood siding differentiates the higher volume from the stucco-clad lower volume. The tall fence provides privacy from the taller surrounding apartment buildings.