I made this sort of castle years ago when I taught at camp. I can't
remember the ratio, but I think it was very forgiving(3 or 4 to 1?). All
you need is clean sand and flour. Mix the flour and sand together dry and
then have the kids add small amounts of water to the bucket until it sticks
together. When they dry they are rock hard. I remember some kids made the
sand mixture into soup and made drip castels. I hope this helps!