"Telling the host at Casita Miramar that you have a reservation at Cuarto 16 feels like uttering a secret password. They will mumble into a headset, then lead you up a stairwell where the conventional trappings of the hacienda-like restaurant give way to a clandestine warren of rooms, open kitchens, bars, and ultimately a tranquil garden terrace above one of San Juan’s busiest enclaves."