Nyctocereus serpentinus is a columnar cactus from the tropical areas of Mexico that tends to trail as the branches get longer.  It prefers a well draining soil mix and adequate water in the summer.  It flowers in the summer and the flowers are very attractive, nocturnal, white with pink outer petals, 15-20 cm long and fragrant.  Nyctocereus serpentinus can actually be trained to grow along a fence and then it is definitely a protection against intruders.