Complex sentence: Henry II of Castile's grandson, Henry III of Castile, married Peter of Castile's grand-daughter, Catherine of Lancaster; their son, John II of Castile, was the father of Isabella I of Castile.
Split into simpler sentences: Henry II of Castile's grandson, Henry III of Castile, married Peter of Castile's grand-daughter, Catherine of Lancaster. Their son, John II of Castile, was the father of Isabella I of Castile.