Caltanissetta's historic centre converges on Piazza Garibaldi, a handsome square flanked by the Duomo, the town hall and the baroque Chiesa di San Sebastiano. Although the Duomo sports a late-Renaissance appearance, substantial alterations made in the 19th century have ruined the overall effect. Inside, if you find the church open, are frescoes by the 18th-century Flemish artist Guglielmo Borremans.