Complex sentence: It also brought a new aristocracy to the country that dominated landholding, government and the church and who brought with them the French language and maintained their rule through a system of castles and the introduction of a feudal system of landholding.
Split into simpler sentences: It also brought a new aristocracy to the country that dominated landholding, government and the church. They brought with them the French language and maintained their rule through a system of castles and the introduction of a feudal system of landholding.