Complex sentence: The Castillo is a masonry star fort made of a stone called "coquina", Spanish for "little shells", made of ancient shells that have bonded together to form a type of stone similar to limestone.
Split into simpler sentences: The Castillo is made of a stone called "coquina", literally "little shells". This is what the stone is made of, ancient shells that have bonded together to form a type of stone, similar to limestone.