"Starting at Katharinenkirchplatz 13, by St. Katharinen church, you'll open the Explorial app, enter the 6-digit game code that you received by text and Viator chat message, and set off through the old town. The tour is fully self-guided — no guide, no fixed start time, just your group and the city. On the Dominsel, the cathedral island in the Havel, riddles lead you around the spot where a Slavic fortress became the seat of a bishopric in 948 — the founding act of the entire region. At the Altstädtisches Rathaus, a late-Gothic brick beauty, you'll size up the 5-metre Roland statue standing guard outside — a medieval symbol of civic freedom, and the answer to more than one clue. Villa Lehmann brings a different era: a grand industrialist's home from the days when the Havel powered mills and factories, with details worth a second look. Most groups finish in around 2 hours, with breaks whenever you like. By the finish, Berlin's older neighbour will feel like your discovery."