Make a giant mountain of sand. Find a level spot near the water but not close enough that the tide will wash it away. Use your shovel or spade to move the sand into a giant heap.
Add water. In order to build a strong sandcastle, the sand must have a cement-like texture. Turn your mountain into a volcano by digging a hole at the very top and pouring water into the hole. The water will seep down into the structure.
Pack it tight. Once the water has seeped down through the structure, press the structure to pack it tight. Level off the top so the structure looks like a flat base.
Start adding towers. Take a wet scoop of sand and add it to your flat base. Shape the tower structure with your hands and pack it tightly. Repeat for more towers.
Shape, perfect and decorate. Once you are done creating the shape of the actual structure, start ironing out the details. Use a spatula to imprint the sand with the look of rooflines. Smooth out all edges and add pretty seashells or even some seaweed to give a nautical look to your castle.