Starting in central Hanoi near the Hoan Kiem Lake area, open the app and enter the 6-digit game code that you received by text and Viator chat message. The lake and its surroundings have been the heart of Hanoi for centuries — the riddles take you deeper. The first clue leads to the Hanoi Opera House, a grand French colonial building completed in 1911 that has staged everything from classical concerts to political rallies and stands as one of the most elegant structures in Southeast Asia. From there the app takes you to St Joseph's Cathedral, a neo-Gothic church built in 1886 on the site of a razed Buddhist pagoda — a collision of histories that Hanoi carries with characteristic composure. Your final stop is Ngoc Son Temple, the Island of the Jade Mountain temple sitting in the middle of Hoan Kiem Lake, reached by the bright red Huc Bridge — one of the most iconic images in Vietnam. Around 2 hours on foot. Carry water and move at a pace that lets you absorb what you're seeing.