Complex sentence: Once he had started to amass his fortune, he built Broneirion, a grand country mansion on the north side of the river that would enjoy sunshine all year round, which is maintained in good order to this day in the ownership of Girlguiding Cymru.
Split into simpler sentences: Once he had started to amass his fortune, he built a grand country mansion on the north side of the river that would enjoy sunshine all year round. This mansion is "Bron Eirion" which is maintained in good order to this day in the ownership of Girlguiding Cymru.