[/quote]No, that is not correct it is a cultivar of Acanthocereus tretragonus, which is a very different looking plant compared to the Fairy Castle cultivar. [/quote]
Have you ever seen true Acanthocereus tetragonus? Forest cactus with long branches.
It's very easy to ID the original species of this cultivar if to cut one bigger branch, deleting all new branches at beginning, plant in bigger pot and give enough water, then it's grow will be faster and stem more thicker with bluish epidermis, at last it will not give any new branches at all and top of the stem will be 100% Cereus.
As me as my friends did it. I grow also 2 opuntias from "monstrose" cultivar same way and got at last O. ficus indica and Austrocylindropuntia subulata.
These cultivars are product of slow phytoplasma infection which cause prolific monstrose growth. In case when stem grow faster, infection hasn't time to spread and top tissue will be free of phytoplasma.