Tonto's roots aren't intentionally vague or ambiguous (-;

I don't think the roots should be a big problem for a well built retaining wall, but it does sound like a pretty tight place, visually, for 'Tonto'. 3' might not be too bad, but less than that would be cramped.

Here is the spot that I have about 1 foot to dig and then I hit ledge. Would it be better for me to plant it in a very large whiskey barrel? Thanks.

Here is a link that might be useful: {{gwi:333271}}

I think what you are saying is that you hit solid rock at a depth of 1'?

If that's the case, you will probably not be able to grow the tree in that area. There just won't be enough moisture in the soil. Seedlings might stand a chance, depending on conditions, because they have a longer time to become established and grow an appropriate root system to deal with the difficult conditions. I would be tempted to find another location in the yard for the tree. It could be grown in a pot, but you'd have to replant ever year or so as the potting soil broke down, and, eventually, the tree would get too big to easily manage that.

Is there a possibility of building a raised bed? Properly designed, that might be one solution. It might also give you a raised area in which to plant smaller plants, but still allow them to be up closer to eye level.