Complex sentence: James Burrell became Surveyor of Works in 1604, making him responsible for maintenance of the bridge, having previously also been occupied on the fortifications around Berwick before James VI and I ascended the throne of England, making them redundant.
Split into simpler sentences: James Burrell became Surveyor of Works of the town in 1604, making him responsible for maintenance of the bridge. He was previously also been occupied on the fortifications around Berwick before James VI and I ascended the throne of England, rendering them redundant.