Roll the die to determine which arc the Boulder starts in.
Unlike Monsters, Giant Boulders are never placed on the board. Instead, they instantly resolve as if they were rolling all the way across the board, starting in the Forest, and destroying everything in their path until they hit and destroy a Wall, Tower, or Fortify token.
The Boulder does not stop until it hits the first Castle structure in its path (a Wall, Tower, or Fortify token) and destroys it.
If there are no Walls or Towers in the starting arc, the Boulder continues rolling through the Castle ring and into the opposite arc. If no Wall or Tower is present in that arc either, the Boulder continues towards the Forest, destroying any Monsters in its path. After resolving any damage, the Giant Boulder token is discarded.