By the late 12th to early 13th century, the original palisade walls and wooden keep had been replaced in stone.[14] The new curtain wall incorporated St George's Tower, which is built of coral rag stone, 30 by 30 feet (9 m × 9 m) at the base and tapering significantly toward the top for stability.[15] This was the tallest of the castle's towers, and is now believed to be a survival from late Saxon times (c.