I have never seen an example of a barrel bound to the base as in your photo but I would imagine that the rope had been tared.
I agree on the rope. As to the rope tying the barrel to the base, me either. So I'm trying to figure out more about that, I've reached out to the Royal Armories curator to see if they can explain it or have any period documentation as to the practice. I've combed through Mullers and can't find anything on it their either, so more work to do to that end.