Blockhaus 13 Curiosity · Colmenar del Arroyo, Sierra Oeste, Madrid Image of Colmenar del Arroyo, not of the place itself. A circular, concentric blockhouse built between late 1938 and early 1939 by the second company of sapper battalion number 7; of the twenty-two bunkers planned to that design, this was the only one ever finished. Photo: Dirección General de Turismo. Consejería de Economía e Innovación Tecnológica. Comunidad de Madrid (CC BY 3.0 es) What to see and do in Colmenar del Arroyo