Using your bucket and or shovel, dig a four to five inch deep trench around the area that the castle will be located. Be sure to pack the sand within the trench tightly. When construction is complete this trench will be filled with water to deter unwanted visitors. For now it will be used to mark your area.
Using moist sand and the flat part of your shovel, pack small mounds of sand connecting the larger mounds of sand. Be creative, the castle does not have to be a perfect square or circle. These walls can be straight or curved, it is up to you. Keep in mind, do not get too close to the moat.