The Siege Perilous is also used in Witch World by Andre Norton copywright 1963 in which she makes it part of Cornish legend, also ties it to the Arthurian Round Table, and uses it as a device that judges the man who sits in it and sends him to the alternate probability world that best fits his character.  There is no return to this universe.  It is used, for a price, by people in desperate need of escape.