You may see Cathedral of Saint Domnius in the skyline before you actually find this important religious building. Look around inside if the doors are open or just take in the facade from outside.
If you're spending a while in Old Town Split, Cathedral of Saint Domnius is a fine place to kick off your sightseeing. This principal church is one of many attractions worth seeing here. Diocletian's Palace, Salona, and Gregory of Nin Statue are also waiting to be explored close by.