The Temple of the Feathered Serpent is located at the southern end of the Avenue of the Dead, Teotihuacan's main thoroughfare, within the Ciudadela complex. The Ciudadela (Spanish, "citadel") is a structure with high walls and a large courtyard surrounding the temple. The Ciudadela’s courtyard is massive enough that it could house the entire adult population of Teotihuacán within its walls, which was estimated to be one hundred thousand people at its peak. Within the Ciudadela there are several monumental structures, including the temple, two mansions north and south of the temple, and the Adosada platform. Built in the 4th century, the Adosada platform is located just in front (west) of the Temple of the Feathered Serpent, obscuring its view.