This would look pretty amateurish and flat without any shading for dimension. Here I have started on the right side.
I used Water-soluble watercolor pencils
I also had to resort to colored chalk for the shading under the arch. These are cheap ones from A.C. Moore.
Here is the shading with three colors of greenish brown colored pencils on the stones. The shadow under the arch is green followed by terracotta, then 2 other shades of green, one on top of the other all in colored chalk.
The arch with "Base Extender" from ProChemical and Dye to wet or liquefy the watercolor pencils as well as make the chalk permanent. Both the pencil and chalk would wash off if wet.
Finished with the base extender and waiting for it to dry. May take all day since we are socked in fog. The wet chalk shadows look much better after "wetting".