Inside of the castle in Timberdale, get to the part with the long narrow pathways in the air. Once you get to the jumping part, drop off the edge to land on a small platform. Jump to the other platform and do a pulse-jump below the shiny spot. You should hit a button that will open a door underneath the normal path. You can dangle from the bottom of the path and climb through the door. You will then be inside the Dragonreach Sandcastle.