Starting at José María Pino Suárez & C. de Venustiano Carranza, Centro, 06060 Ciudad de México, CDMX, Mexico, open the Explorial app and enter the 6-digit game code that you received by text and Viator chat message. You're in the historic centre of one of the world's great cities — the riddles put it in context. Your first landmark is the Palacio Postal, the ornate Baroque-Moorish post office completed in 1907 during the Porfiriato era, whose gilded ironwork staircases and marble floors make it one of the most spectacular public buildings in Latin America — and one of the least expected. From there the app takes you to the Palacio Nacional on the Zócalo, built on the site of Montezuma's palace and Hernán Cortés's residence, whose Diego Rivera murals depicting the history of Mexico from pre-Columbian times to the revolution are among the greatest public art works in the world. The route concludes at the Casa de los Azulejos, the House of Tiles, clad in blue and white Talavera tiles — one of the most beloved facades in the centro histórico. Around 2 hours on foot. The Zócalo area rewards slow walking and repeated looking up.