I was on the card that this happened on. Neato.
Here are the facts of the situation:
First off. The disc was 100% surrounded by concrete.
Second. We took pictures of the lie and played the shot both ways. (inbounds/out of bounds)
Third. The retaining wall and sidewalk were flush surfaces. The fence that the disc was leaning against was bolted to the same flush surface. If you unbolted the fence and removed it, it would not be obvious that the side walk and the retaining wall were two separate things.
So, those are the facts.
Here's my opinion based on seeing it first hand:
First. It was crazy and I don't think I'll ever see anything like it again. The wind was blowing steady 25 which pinned the disc against the fence vertically and it just leaned there when it came to rest.
Second. The OB was on or across sidewalk. There was no mention of retaining wall. But, my interpretation, personally, was on or across concrete. And again, most people would have considered this retaining wall part of the side walk anyway.
Having TD tournaments in the past, I would have ruled it OB and it would have been and easy call to make.
Any chance you can post a pic for those not envisioning the specifics?