Tours exploring the 17th-century Swedish-built tunnels connecting the bastions that ring the town walls depart from the Kiek in de Kök tower. Over the years, they've been used as fallout shelters, homeless refuges and punk rehearsal spaces. Bookings are required, and warm clothes (it's about 10°C, or 50°F, down there) and sensible shoes are recommended. Regular tours finish in the Carved Stone Museum, showcasing tablets, statues and other historical lapidary work from Tallinn. Combined tour and tower tickets are available (€13).
Special tours of the Swedish bastions, entering otherwise off-limits sections, are offered to large groups (adult/child €6/3.50).