Treasure!

Conscious of the noise your splashing oars are making in this silent place, you start to row. As you skirt round the outside walls of the castle you notice a rusted metal grille a few inches above the level of the moat. The boat is unsteady and the metal bars are stuck more firmly than they appear, so you wrestle with one with some difficulty. Eventually, it comes loose in your hand, sending a sprinkling of dust and small stones into the water. You reach inside the dank recess in the wall and pull out a sparkling crystal.