Complex sentence: The riches from the Americas came through Castile was one of the more dynamic, rich, and advanced territories in Europe in the 16th century and started to realise that it could become immersed within an empire.
Split into simpler sentences: The riches from the Americas came through Castile which was one of the more dynamic, rich, and advanced territories in Europe in the 16th century. It started to realise that it could become immersed within an empire.