Sandcastle
A sandcastle is a small castle made of sand. It is a decent option to lay the foundations of your kingdom, though this may prove to not be the best idea, as sandcastles are prone to getting destroyed by the ocean waves. However, there is a way out of this. You can enlarge your castle and replace sand with sandstone, making for a good beachside fortress, as long as you stay on the lookout for opposing ships. Some might have cannonballs that can turn your castle to sandy ruins!
Only a master sandcastle builder will add a sandstone wall and a large moat surrounding the sandcastle. Some may even think about replacing the water in the moat with lava, poison, or acid.