While most sand castle contests forbid the use of glue or cement in the creation of the sculptures, amateurs can enjoy longer-lasting sculptures by creating a mixture of sand, glue and water. Common household white glue will work as both an in-sand cement base and a finishing spray. In hot conditions, you will need to work in small sections with your sand/cement mixture to ensure it doesn't dry before you mould the shape.
Build the base of your sand castle. You do not need to use the glue in the base if it is a square or rectangular shape. The weight on these base shapes will hold the sand together with only water added to the sand.
Pour 10 cups of sand into a large bucket with three cups of water and one cup of white glue. Mix thoroughly. If you are unable to mix the components easily, add water until you can push the sand around the bucket and the mix can be moulded.
Add sections to the castle base from the bucket, using smaller boxes, cups and cones. Sculpt one bucket of sand at a time before the glue dries. This time period will vary based on weather conditions and the size of the bucket. Add small amounts of water to the bucket if you need to keep the sand and glue from drying prematurely.
Mix more buckets of sand, glue and water as needed. If you are using more than one bucket for a section prior to doing the sculpting, work from the inside of the structure out, piling several buckets of just wet sand before getting to the outer layer where you will sculpt. Remember that the weight of the sand in a base-structure is often more than enough to hold the sand together.
Mix one part glue with two parts water in a spray bottle. Spray the completed sculpture with the glue/water mixture to create a layer, protecting the castle from wind and light rain. To test that you have applied enough to the surface, blow on one section to see if the glue is holding the sand in place.
The larger the sand castle base, the more sturdy it the structure will be.