Complex sentence: Magnus was based on Mann during his time in the West, and he had a number of forts and houses built on the island, probably also gaining suzerainty over Galloway in the process of getting the building supplies.
Split into simpler sentences: Magnus was based on Mann during his time in the West, and he had a number of forts and houses built on the island. He probably also gained suzerainty over Galloway in the process of getting building supplies.