Am I too late to offer a suggestion? Buy some landscape cloth and strech it across the back side of the stones so the soil won't wash out but water can drain away.
You will have fun planting that new area, no? Have you saved some chopped leaves in bags? If so, mix them in with the topsoil they deliver. It will be very helpful when the leaves break down, it will be like a composted soil in your new area. You may already do this so forgive me if you are aware of this. Good luck.
Trowelgal

Hello Trowelgal!
Thanks for your answer! No, you are not too late...the dirt was just delivered...
DH will be getting ready to do prepare it this week. I will tell him your suggestion. He has used the landscape cloth to keep weeds down in the old garden, so I know he will be getting more for this one, if he doesn't have any.
He is the gardener...I help pick out plants, but he is the one with the green thumb!
Have a great day!

Hi,
How is your walled garden coming along? What part of Mo. are you in? I am in the Kansas City area and we have had plenty of rain here. I have a castle wall bed too. It is only four stones deep and the contractor put in a special drain. It is the length of my house and I have a lot of Hydrangeas in that bed. It used to be a washed out area with a holly at each end and a butterfly bush off center. The hollies were so overgrown that I pruned them into trees and they look so cool! Moved the butterfly bush to the other side of the house. Along the edge of the wall I have a sedum that cascades over the edge and looks so neat. But it didn't happen overnight, took about three years to get it that mature. I used to hate that area and now I love it!
Hope you can create a beautiful area that you'll be happy with.
Tina