Hmmm, that's interesting.
Firstly, I agree the side wall reference was incorrect.
That said, if you Google "
were 26 acrosses used to reinforce (or walls of old" then it says yes, but if you Google "were 26 acrosses used to reinforce the walls of 2a" then it say the answer is no.
So my reading is that it is a DD of either "they reinforce walls" and "old city" or "they reinforce walls of old" and "city" but that second one seems unlikely as, even though the city still exists, it is one of the oldest in the world.
I could be wrong, of course, but then where does "they reinforce walls" fit into the parsing?