Answer: Most trees send out roots well beyond the spread of their branches, so depending on the width of the planting space and location of the retaining wall, this may not be the best spot to plant a tree. There is an upright in youth but pyramidal with age form of European Hornbeam and it is a very slow grower, so that might be a consideration, but it does reach 30 to 50 feet high with age so it is not really a small tree. Here are a few trees that would be smaller than 20 feet: look at named cultivars of Amelanchier; smaller, disease resistant cultivars of crabapple such as Malus "Indian Summer"; Japanese tree lilac (Syringa reticulata); Cornus mas; smaller cultivars among the deciduous magnolias. I hope this gives you some ideas.
Q&A Library Searching Tips