The centrepiece of Cathedral Island, this three-aisled Gothic basilica was built between 1244 and 1590. Seriously damaged during WWII, it was later rebuilt in its original Gothic form. Entry to the church is free, but you need to buy a ticket to visit three beautiful baroque chapels, and to ascend to the viewpoint atop the 91m-high tower (there's a lift, but check the website to make sure it's open when you visit).