The Azadi Tower, a memorial built under the reign of the Pahlavi dynasty, has long been the most famous symbol of Tehran. Originally constructed in commemoration of the 2,500th year of the foundation of the Imperial State of Iran, it combines elements of the architecture of the Achaemenid and Sassanid eras with post-classical Iranian architecture. The Milad Tower, which is the sixth tallest tower and the 24th-tallest freestanding structure in the world, is the city's other famous landmark tower. Leila Araghian's Tabiat Bridge, the largest pedestrian overpass in Tehran, was completed in 2014 and is also considered a landmark.