By the way, this is my first instructable! :D
Step 1: Creating a "Swand Pit"
Step 2: Foundation
Ok, randomness out of the way, we must start building the castle foundation.
get a big handful of sand and put a plop of it down on the beach, Near your Swand Pit. Not to close though! We don't want the castle collapsing! Build it up and out. Make it as wide as you want, but don't make it too tall! We still want to make towers!