since I do not know where you are located the first thing I'd recommend is contacting you local building department to inquire about this. I know in my area to construct a concrete wall in the edge of a lake I'd need permission of the local conservation commission.
to have uneven backfill you'd need a concrete retaining wall. Depending on the unbalanced height most building codes require this to be designed by a professional engineer. Again, another question for your local building department.
"You get what you pay for, and sometimes free costs more!"