The Castle of Mytilene dominates the northern headland of the city, its walls enclosing a peninsula that separates the two ancient harbours. The fortress was built largely by the Genoese Gattilusio family in the 14th century on foundations that go back to antiquity, and extended by the Ottomans after they took the island in 1462. Walking the ramparts gives sweeping views over both the north harbour toward Turkey and the south harbour down into the city. Inside the walls you'll find the remains of a Byzantine chapel, a mosque built after the Ottoman conquest, and an aqueduct. The site is atmospheric rather than heavily curated — bring water and wear sensible shoes for the uneven ground.