The patron saint of Casteltermini is Saint Vincent Ferrer, celebrated on 5 April, with the cathedral dedicated to him being the town's main place of worship, in Sicilian Baroque style, built on a rocky knoll in Piazza Duomo. To ensure the seriousness of his promises, the baron wanted the pacts between lord and vassals to be set down in a public deed: thus on 5 April 1629, the feast day of Saint Vincent Ferrer, who belonged to the same ancient family as the baron, the noble house of Ferreri of Valencia, he gathered the town's notables and, before the notary Pietro Chiarelli da Sutera, signed the «Capitoli della Terra», granting favourable conditions to the new inhabitants.
Casteltermini reached its peak development in the second half of the 19th century thanks to the area's numerous sulphur mines, including Cozzo Disi, Mandravecchia, Roveto, San Giovannello and Scironello: on 4 July 1916, at the Cozzo Disi and Serralonga mines, one of the most serious workplace disasters in the entire history of Italian mining occurred, killing 89 workers. The town is also known for the Festa di Santa Croce, whose current form dates from 1667, its origins tied to a legend of a found cross, now dated by scholars to between 1634 and 1665.
