The city's broad central avenue, called "Avenue of the Dead," is flanked by impressive ceremonial architecture, including the immense Pyramid of the Sun (second largest in the New World after the Great Pyramid of Cholula) and the Pyramid of the Moon. Along the Avenue of the Dead are many smaller talud-tablero platforms. The Aztecs believed they were tombs, inspiring the name of the avenue. Now they are known to be ceremonial platforms that were topped with temples. Further down the Avenue of the Dead is the area known as the Citadel, containing the ruined Temple of the Feathered Serpent. This area was a large plaza surrounded by temples that formed the religious and political center of the city. The name "Citadel" was given to it by the Spanish, who believed it was a fort.