From the World Heritage inscription:
The Cathedral of Šibenik is the fruitful outcome of considerable interchanges of influences between the three culturally different regions of northern Italy, Dalmatia and Tuscany in the 15th and 16th centuries. These interchanges created the conditions for unique and outstanding solutions to the technical and structural problems of constructing the cathedral vaulting and dome. The structural characteristics of the cathedral make it a unique and outstanding building in which Gothic and Renaissance forms have been successfully blended.
Šibenik is a small town on the Dalmatian coast, opening out on a bay separated from the Adriatic by the Sveti Ante (St Anthony) channel and a multitude of tiny islands. The town was founded in the 10th century by the Subic family; it consists of a labyrinth of narrow streets and small squares climbing from the level of the cathedral to the fortress at the summit of the old town. Early in the 12th century it came under the sway of the kings of Hungary, who granted its independence. In 1116 and 1378 Šibenik suffered at the hands of the Venetians. They took the town in 1412, renaming it Sebenico and holding it until the fall of the Venetian Republic in 1797. The cathedral of St James owes its present appearance to three successive periods of construction between 9 April 1431, when the first stone was laid, and 1535.
Georgius Mathei Dalmaticus built the cathedral, with the exception of the nave and the aisle walls, by assembling slabs of stone and the contiguous sections of pilaster and ribbing using a particular technique for the joints. The roofing of the aisles, as well as that of the apses and the dome, is made from stone ’tiles’. These roofing tiles are laid side by side with their horizontal edges overlapping, and the joints are made by the perfect fit. On the dome the tiles are held in place by stone wedges fitted with great precision and are inserted into the ribs as into a portcullis. This type of construction could well have taken its inspiration from shipbuilding, or from the experience of many artists whose first trade was the working of wood as joiners, cabinet-makers, or model makers. The solution adopted for the cathedral at Šibenik was made possible by the outstanding quality of the stone used, which came from the stone quarries of Veselje, on the island of Brac, which are still in operation to this day.
St. James’ Cathedral isn’t the grandest or most beautiful cathedral in Europe let alone Croatia. It’s inclusion on the world heritage list seems out of place compared to other great and historic cathedrals which have been listed.
The real reason I think St. James Cathedral is on the world heritage list is its method of construction. There was no mortar used in the walls. The stones were carefully cut and stacked without anything holding them together.
There are several great world heritage sites in Croatia. However, I don’t think St. James’ Cathedral is one of them. If you are in Šibenik or driving down the Dalmatian coast it is worth a visit (you can probably experience the whole in the 15 minutes) but only the hardcore world heritage site fan is probably going to find it of interest.