Castle builders can just create. There are no plans to follow. Symmetry is not as critical in castle sculpting as it may be in creatures or faces. Turtles, fish, dolphins, sharks and whales are favorite shapes. Try a sea monster or a dragon, or maybe a lizard type thing.
Cars are a great item to shape.
Rough it out first. The Sand Wedge Hoe is for roughing out your form. It will also remove the excess sand from the project as you work. The baby shaper is good for minor details, like writing in the sand. The Papa Shaper is great for shaping the larger features or forms that will become the key points of your creation.