It is known that the first sanctuaries were created shortly after the Nightmare Wars began by the great master dreamwright elder Klin Benfar. He created the first sanctuaries within Underlight as a foothold from which to stand against the destructive tide of marauding Mares during the Nightmare Wars.
Building upon Klin's knowledge used to create the first Sanctuary, Master Dreamwrights of the Gathering created the Sanctuary within the House. This was created within an existing room between Gathering Hall and Center Hall, the original purpose of the room has been lost to antiquity.
The physical construction vaguely resembles the "infinity" sign "8". Each side is exactly like the other, or a mirror image as it were. Each with a small round pool, surrounded by a circular walk, each having 5 supporting pillars on the outside wall. Directly between these two sections of the room a small raised dais, the center or fulcrum point of balance. The color scheme of this sanctuary is that of most of the House. Greens, Browns and Grays, a representation of the "physical" world between Chaos and Nirvana.
This sanctuary had many purposes, it served as a redoubt from the from the mares, and since no violent actions could be performed within Sanctuary, it served as a place of mediation as well as meditation.
The Sanctuary's in Upper and Middle Basin were constructed concurrently, some time after the Sanctuary at the House. Their construction was similar to the Sanctuary of the Gathering, differing slightly, as shown by the vaulted columns in the center of each pool, but their coloration was drastically different. Based upon the underlying theme of balance and Harmony, each of these two Sanctuaries were colored to represent oppositional purposes and symbolisms.
The Sanctuary in Upper Basin. Here the coloration is dark, with the symbolism of stars, (in groupings of 5), all about it's interior. It is believed that here, as well as being a redoubt from conflict, many meditate and reflect on the balance and harmony of actions and events external to the Dreamer, symbolized by the Night and Stars. To reach out beyond oneself to the possibilities and knowledge that can be gained from contemplation of the physical aspects of each plane. This external focus is also further emphasized by the other places within the Upper Basin, "the Commons" and the "meeting rooms". All with external connotations and emphasis on conducting business and discussions on the physical plane.
The Sanctuary in Middle Basin. Here the coloration tends toward light white colors. A redoubt as well, it is believed that many come here to meditate and reflect inwards. To gain knowledge and insight concerning the spiritual aspects of the Dreamer. This sanctuary and it's purpose is opposite to the one in Upper Basin, together they are the poles of thought, inner and outer, between which the harmony and balance exist. As with the Upper Basin, the other places within Middle Basin, such as Fayd's Fortress, the Study, Meditation Hall and the Private meeting room all reflect spiritual and inner purpose and reflection.
Although each Sanctuary, has as it's basis, the normal functions associated with all sanctuaries, it is believed that those in the Gathering and the Basin, were built with an underlying purpose within the Beliefs, The Charter, and the Oath of the Gathering. As a diplomatic House, the Gathering offers Sanctuary to many in their time of need, and a non-violent balance, or solution to conflicts is pursued and implemented. These Sanctuaries were built to help and ensure the Gathering could do so in a location where violence could not occur.
As to why there is no Sanctuary in the Lower Basin of the Stars, one can only assume. At the time the Sanctuary's were built, Many of the lands of the Lower Basin (The Stairs and Rooms, as well as the area of the Fields) were already established and set aside before the Nightmare Wars. Each with it's own purpose, but a more justifiable reason no Sanctuary was built, in this Dreamers reasoning, would be the fact that Cholok's Temple is located within the Lower Basin.
There is much symbolism that can be found within the Sanctuaries that can be closely associated with the Beliefs of the Gathering, The Avatar Crest and the Pillar system, but these are only theories, that I will discuss separately.
-Yphymbly