I have a small area behind my house that I want to update. The issue is that there is about a 6 to 12 inch height difference between my property and my neighbor's, and this height differential is right under our shared fence. Looking at the pictures, you can see that I can hardscape right to my house easily, with river stones, pavers, etc. But the left side needs some sort of border; something to 'contain' my landscaping, for lack of a better term. Ideas so far:
a) put treated timbers right under the fence. The timbers would be visible on my neighbor's side, and on my side they could contain the rocks (or whatever) I decide to put down.
b) Build a short retaining wall with stones, on my neighbor's side. Not sure this would serve as a good border on my side.

Also, I'm not sure what to do with that messy gap between my hot tub and this fence. In retrospect I should have taken my pavers and border right to the fence?? I'd love some general advice on what to do in these 'gaps', where landscaping meets house, or fence, etc.

I included a pic of the front of my house, to give the general idea of what I'm trying to create.

Thanks so much!
Carter