I have a home with a retaining wall about 8' from the back of the
house. The retaining wall is built with treated timbers and is about
4' high. I would like to build a deck on the second level of the
house, facing the retaining wall. How should I support the deck?

Is it better to set the posts (1) on the higher ground beyond the
retaining wall, (2) on the retaining wall itself, (3) on the lower
ground short of the retaining wall, and if so should it touch the
retaining wall (flush) or not?

Ideally, on the retaining wall or on the higher ground beyond the wall
would be best use of space, so please let me know if those options are
feasible, even if not the best plan. Thanks.