The three primary steps necessary to build a sand castle include securing a source of water, creating a large base and keeping the sand moist while building. Securing buckets, carving tools and shovels is also necessary.
Building a sand castle requires a great deal of water throughout the process. To ensure that there's a good water source available, builders can dig a hole into the sand to the water level at the beach. Locate the hole near where the castle base is being built to ensure that there is a continuous supply. For non-beach castles, keep a medium-sized bucket full of water nearby.
Build a base of hard-packed sand that is slightly larger than the planned castle to provide a level building surface well above the water level. This also helps to drain water from the sand used to build the castle. Pile the base sand a few inches in height, making sure it is wet, then pack it down tightly to create a solid base.
With the base in place, start building the castle structures with wet sand. This is done by piling a large mound of sand onto the base, placing holes in the pile, filling the holes with water and mixing the sand in place. This can also be done by mixing the sand with water in a second bucket and dribbling it onto the base where the structures will be built. Shape the wet sand into the desired structures, then carve in the details. Make sure the sand remains moist while carving by adding water when necessary.