PlantFiles: Cereus
Cereus hildmannianus subsp. uruguayanus 'Fairy Castle'

Seem to be rather common in the succulent part of the box retail stores.
Additional information - they are very susceptible to a disease that often cause their tops to rot, spreading to the rest of the plant.

A very interesting cactus. Mine likes filtered light. The mature arms start to turn red or tan when subjected to direct sunlight for a day's duration.

A very positive experience.One of the first cacti i ever bought and still have it 25 or so years later. Always in a pot but it could be ground safe. Likes to be consistently fertilized during the growing season.The only caveats is that snails and slugs will ruin it muy pronto. Indoors watch for mealys. A tough little plant.
2012: wow,Seven years since I wrote that..and only today has it gone in the ground. And it is true-they are susceptible to a rotting virus...mine has lost much of its deep green color,and few new branches. Planting it out is a last resort. It is a mealy magnet..so no doubt they spread the illness.
I have strong hopes that with roots in Ma Earth,the Castle will rise again!

This was for the longest time thought to be a Monstrose or Dwarf form of Acanthocereus tetragonus in the horticultural world until one day it was discovered (by me) a new generation from the parent plant which is Cereus hildmannianus ssp. uruguayanus. You can see the proof in this thread here: http://davesgarden.com/community/forums/t/1031261/
I quickly did a trade with dixiegril to have my own specimen of the Dwarf form, and these photos here are photos of that specimen.
The'Type' of spines are not even the same 'Type' of spines of Acanthocereus tetragonus which you can compare with this photo here: http://davesgarden.com/guides/pf/showimage/171302/
There are also hairs on the areolae of both the Parent Uruguayanus subspecies and also on the 'Dwarf' form, but not on the Parent Acanthocereus.
The rib width and depths are different also than that of Acanthocereus tetragonus
I hereby declare the origin of this Form solved by me.
(Edited Oct 4, 2010)
Xenomorf.

This is a very interesting little plant. Here is a good description given by The Cactus Collection/Altman Plants:

"Cereus tetragonus cv. 'Fairy Castle' is charming miniature form of Cereus tetragonus. The type species, native to Brazil, is a branching green columnar cactus to 6' in height. A cultivar occurs when an environmental or other factor causes changes in a plant. A cultivar is not a hybrid, it is rather a change in the DNA of the cells of one unique plant. The diminutive cultivar 'Fairy Castle' was discovered in cultivation, and will grow slowly to several feet in height. The main stem is five sided, with short spines along the ribs, and offsets profusely to form smaller upright branches which, in turn, offset profusely with smaller offsets. Overall effect is of a "castle" shape, with literally hundreds of branches and branchlets. Prefers warmer winter temperatures. Porous cactus soil with adequate drainage. Bright, filtered light with ample airflow. Water thoroughly when soil is dry to the touch. Protect from frost. "