You will enjoy having ground covers to keep out the weeds. Some that grow in the shade include a few varieties of lamium, sweet woodruff, ajuga, and vinca minor. There are some that are very aggressive, such as moneywort, aegapodium ( snow-on-the-mountain), and even wild violets. Cleome or spider plant is a tall plant that would give you height and color in the center of your circular garden, and it reseeds every year. Two perennials might interest you as well. Macleya, commonly called plum poppy, has a bloom that looks nothing like a poppy. It towers to 4 to 5 feet and has very interesting scalloped foliage, but it does spread. Joe-Pye weed is a tall plant that blooms in late summer, and the bees and butterflies love it. An ornamental grass might be a nice possibility too, such as the variegated Miscanthus sinensus variegatus that grows up to 5 to 6 feet tall.