Complex sentence: Henry Walters assembled a collection of art produced during the Middle Ages in all the major artistic media of the period, and this now forms the basis of the Walters' medieval collection, for which the museum is best known internationally.
Split into simpler sentences: Henry Walters assembled a collection of art produced during the Middle Ages in all the major artistic media of the period. This forms the basis of the Walters' medieval collection, for which the museum is best known internationally.