Complex sentence: The River Avill was important for trade, the region around Dunster was rich with fisheries and vineyards and Dunster Castle prospered and stone fortifications were built on the site during the early 12th century, probably forming a shell keep around the top of the motte.
Split into simpler sentences: The River Avill was important for trade; the region around Dunster was rich with fisheries and vineyards, and Dunster Castle prospered. Stone fortifications were built on the site during the early 12th century, probably forming a shell keep around the top of the motte.