Complex sentence: Note that the town of Kaštela is a unique exception in that it only exists as an administrative unit-it is legally treated as an agglomeration of seven separate settlements, none of which is called "Kaštela".
Split into simpler sentences: Note that the town of Kaštela only exists as an administrative unit. It is legally treated as an agglomeration of seven separate settlements, none of which is called "Kaštela".