This diminutive cultivars 'Fairy Castle' was discovered in cultivation and is a very popular plant often found in the succulent part of the box retail stores.
Some expert classify the cactus as a monstrous form of Acanthocereus tetragonus (Acanthocereus floridianus), but the true self of this plant is controversial and some suggest that it may be a form of Cereus hildmannianus or even a dwarf form of Cereus hildmannianus subs. uruguayanus.
Whichever scientific name is correct, the plant is a delightful cactus.