the spiral they show in Gaia's garden just had stones set every so often up the spiral. Even mentioned putting a pond, um, somewhere around the base. But I wonder if building and using mortar won't somewhat limit the ability to share and spread nutrients. I know that certain plants add or take certain things from the soil so that planting diversity sort of evens this out. Also their design has a pvc pipe running from under the base up through the middle to facilitate watering (an "L" shaped pipe presumably with threads on the lower end to hook up water. Just wondering.