The Plaza of the Three Cultures in Tlatelolco is far and away the most famous part of the neighborhood. The site of recent tragedy, and ancient intrigue, it's also among the most visited parts of Mexico City.
The "Three Cultures" in the name refer first to the ancient period evident in the archaeological digs that are ongoing in the area. The second is the Spanish colonial period, evident in the stark and austere Church of Santiago Tlatelolco. Third is of course, the turbulent modern Mexico City of architect and urbanist Mario Pani whose reputation rose and fell here.
The Three Cultures Plaza is quite big in and of itself. It's also an integral part of a much bigger neighborhood, and of Mexico City, too.