Just around the corner, this is a big hang-out place. There are interesting plaques on the low walls around the square, which describe the local history, particularly the canalization of water from local springs carried out by the Moors.
The church is San Andres, where Isabella and Ferdinand attended Mass when they were in Madrid. (There’s a plaque on the wall of the church that fronts on to Costanilla de San Andres.)
The Museo de San Isidro is beside San Andres.
Los Carros (“the carts”) is so named because this is where people parked their carts when bringing goods to be sold in Madrid. It adjoins Plaza de la Cebada, which was the barley market, and Plaza de la Paja, on the other side of San Andres, was the hay market.