Question: Who was the reigning king when the Builth Castle was built and who was also known as "Malleus Scotorum"?
Sentence (from "Saint Margaret of Scotland"): She was a very pious Roman Catholic, and among many charitable works she established a ferry across the Firth of Forth in Scotland for pilgrims travelling to St Andrews in Fife, which gave the towns of South Queensferry and North Queensferry their names.
Relevant evidence: No