Complex sentence: For his part in the potential marriage alliance, Henry II transferred the castles of Chinon, Loudun and Mirebeau into John's name; as John was only five years old his father would continue to control them for practical purposes.
Split into simpler sentences: For his part in the potential marriage alliance, Henry II transferred the castles of Chinon, Loudun and Mirebeau into John's name. As John was only five years old, his father continued to control them for practical purposes.