The Cattedrale di Santa Maria della Purificazione is the oldest building in Termoli and the most rewarding piece of architecture in Molise. It was built between the 12th and 13th centuries on the foundations of a much earlier basilica, and the façade is one of the finest pieces of Apulian-Romanesque work on the Adriatic coast — three round-arched portals, blind arcading above, and a rose window that survived the 1566 Turkish raid which destroyed much of the rest.
Inside, the basilica plan is austere and beautifully proportioned. The crypt below holds mosaic fragments from the original 5th-century church and the silver reliquaries of Saint Timothy and Saint Basso, the city's patrons. Entry is free; the cathedral sits on a small piazza at the heart of the Borgo Vecchio, a minute's walk from the castle.