The cathedral dedicated to Sant'Erasmo (also known as Saint Elmo, patron of sailors) stands at the heart of the upper town and has been the spiritual centre of Gaeta since the early medieval period. The current building blends Romanesque foundations with later Baroque additions — a common fate for Italian churches that survived long enough to attract wealthy patrons in the 17th century.
The interior holds several significant works, including a Flemish polyptych and an Easter candelabrum from the 12th century — a marble column carved with reliefs of the Passion and topped with a candleholder, a rare surviving example of medieval liturgical furniture. The separate bell tower, rising from the piazza beside the cathedral, is the most photographed structure in the old town: a layered Romanesque campanile decorated with ceramic plates and built from reused Roman stone.