The golden rule of Castle is: Make Castle Fun (for You and for Other People). Pretty much everything that's encouraged and disallowed can be derived from this principle.
Some examples of things that you can do that are awesome
Some things you shouldn't do here:
Castle is new and small right now, and so we don't want to make a big list of rules or anything right now; but we think if everyone tries to follow this one, it will help make Castle into the healthy world full of fun and diverse, interesting things that we want it to be.