The Twelve Guardians are explained to guard the Twelve Portals. Each Guardian matches up with a Guardian on the other end of the Beam which they guard, and at the point where all the Beams meet is the Dark Tower. The Guardians are revealed in several scenes, the first is when Roland is explaining to Eddie and Susannah Dean what Shardik was and his understanding of the Beams, another is when Eddie and Susannah approach the Cradle of Lud, and the last, Bird and Hare, are revealed in the poem Roland and Susan Delgado (his lover) recite to each other occationally: 'Bird and Bear and Hare and Fish . . .' The only pairs identified are by Susannah and Eddie while at the Cradle of Lud.
Shardik was created by North Central Positronics, Limited. One theory I have is that the Beams, the Guardians, and the Tower were created by branches of N.C. Positronics as a combined venture to hold the world together after, or while, society wreaked havoc with nature. The beams, however, do more than just hold Roland's world together, they extend throughout ALL worlds. Therefore, it is more plausable that the beams were created so that there would be a point at which they would all cross that would lead to all other worlds. I believe that the Great Old Ones used the Dark Tower to leave Roland's world after doing whatever it is that caused the Waste Lands and the mutations in so many creatures. The Great Old Ones left the Dark Tower standing either so that they could use it to continue to travel to other worlds, or because they were in such a hurry to leave Roland's World. The Great Old Ones probably did not build the Dark Tower for the purpose of leaving Roland's World, but made a B-line for it when the shit hit the fan. The Tower is the lynchpin of all worlds/universes, connecting them together--yet keeping them separate from one another.
The pairings we can be sure of are:
The Twelve Portals and Guardians
The Rose
The rose is in fact the manifestation of the Dark Tower in Jake's where and when. It should be noted that while both Eddie and Susannah appear to be from the same where, such appearances are deceiving. Much like the Ages of Myst, within the infinity of worlds linked by the Tower, anything is possible.
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