La Ciudadela — the 'citadel', a name given by sixteenth-century Spanish chroniclers who mistook it for a fortress — is the great walled enclosure at the southern end of the Avenue of the Dead. It is laid out as a vast sunken plaza four hundred metres on a side, with the Temple of the Feathered Serpent rising from its centre. The platforms around the perimeter are crowned by small temples; the whole space could hold one hundred thousand people, which is roughly the population of the city at its peak.
It is one of the best places on the site to understand the scale of Teotihuacan as a political theatre. Walk around the platform tops for the best view back over the Avenue and the Pyramid of the Sun. The site museum and the new Museum of Teotihuacan Murals are both nearby; combine them as you enter or leave the site.