Gather some reference images to guide you as you sculpt the castle (see Resources). Use a board as the base for the miniature. Use about 2 pounds of clay for a small sculpture. Knead the clay as you would knead bread dough, until the clay feels consistent.
Form the clay into a ball and place it on the wood. Push in on the sides with your palms until the clay forms a cone. Etch a line around the cone to separate the top third of the clay (top of cone). The top will be used to form the castle.
Texture the rock below the castle using real rocks. Press them into the clay to transfer the rock texture. Use different sides of the rocks to create variation as you work your way around the entire piece.
Shape the rough outline of Hogwarts castle. Begin to form the buildings with any utensils you may have lying around, such as small pieces of wood or plastic in addition to your tools. Stamp, shape and remove the clay one section, or building, at a time.
Form the basic shape of all the buildings using the ruler or other straight-edge to shape the walls and rooftops. Some structures are round, while others are rectangular.
Detail the buildings, including pillars, windows and doors. Sculpt the overhang of roofs over the side walls, and as much of the architectural detail you wish to recreate. Experiment with different objects such as Lego pieces that may help you stamp in the areas for the windows.
Remove sharp edges or tool marks using a damp sponge. Mix a piece of clay about the size of a grape with a little bit of water to create a clay slurry. This will act like glue and let you attach pieces of clay together.
Finish the main buildings and walls, including the detailing. Use grape-sized pieces of clay to roll out the pointed towers of the castle. Roll them back and forth on the table and apply pressure on one end. Keep forming this way until you get the right shape for each pointed tower.
Cut off the piece you need with a knife. Apply some slurry on the bottom and place it on the spot it belongs in the castle. Handle these carefully so they retain their shape.
Clean any tool or hand marks with a wet sponge. Use the sponge to finish the rock texture as well, giving the rocks a weathered look. Let the castle dry for several days. Use diluted acrylic paint to stain and paint the castle until it looks finished.