Complex sentence: Baldwin was also asserting his ecclesiastical authority over the Welsh bishops, especially when he made a point of celebrating mass at all the Welsh cathedrals; he was the first Archbishop of Canterbury to celebrate mass at St Asaph's Cathedral.
Split into simpler sentences: Baldwin was also asserting his ecclesiastical authority over the Welsh bishops, especially when he made a point of celebrating mass at all the Welsh cathedrals. He was the first Archbishop of Canterbury to celebrate mass at St Asaph's Cathedral.