There's a huge 19th-century fortress at the mouth of Mahon's vast harbour and it's one of the top things to see in Mahon according to TripAdvisor members. Fortalesa De La Mola (also called Fortalesa Isabel II) is a great place to go if you like exploring as it's very well preserved. Set aside at least half a day for your visit, including your drive or water taxi ride from central Mahon, and take a picnic if you're going to be there over lunch time; we can guarantee you'll need the energy. There are tunnels to walk through, an underground art gallery to study, plenty of above-ground fortifications to see, and some massive cannons. Don't forget your camera either; the views from the fortress are beautiful, whether you're looking out to sea, back to the harbour or along the protected La Mola peninsula, which is a haven for migratory birds. To see the fortress from the water, take a harbour cruise or glass-bottomed boat trip from Mahon. You'll even get to hear about the history of the port while you take in the scenery, so you can relax and learn at the same time.