Complex sentence: At the age of ten, he was apprenticed to the famous sculptor Lorenzo Ghiberti, designer of the doors of the Florence Baptistery, whose workshop was the premier centre for Florentine art at the time.
Split into simpler sentences: At the age of ten, he was apprenticed to the famous sculptor Lorenzo Ghiberti. Ghiberti was the designer of the doors of the Florence Baptistery and his workshop was the premier centre for Florentine art at the time.