Beams are physical forces that hold up the Dark Tower. Like the Dark Tower, they are infinitely old, created at the beginning of the Universe and are themselves living entities (all male and brothers), having arisen from the Prim. The Beams are represented by 12 figurative "animal guardians". In all there are 6 beams and 2 guardians on a beam: bat, lion, bear, horse, rat, wolf, hare, eagle, turtle, dog, fish and elephant. At the end of each beam where a guardian resides, there is also a portal to another world. The Old Ones sought to replace the magical beams with their own technological versions and North Central Positronics created actual giant cyborg guardians.
Though there are 6 beams, in effect there are 12 spokes - the Tower is at the centerpoint of each beam, and an animal guardian at each end. The spoke one is traveling can be described using the animal names in the following manner - the beam that goes bear - tower - turtle, when on the spoke between the bear and the tower, you are on the beam of the bear, path of the turtle. When between the turtle and tower, it is the beam of the turtle, path of the bear - even though the beam of the turtle, and the beam of the bear are the same beam.
'All things serve the Beam' is a common theme throughout the story. Once Roland, Eddie, and Susanah reach the portal of Shardik, Roland demonstrates the power of the beam through using a crude compass. The ka-tet are able to see the beam at work, by seeing birds turn to follow the beam as they pass through it. Trees grow in the direction of the beams, even their shadows follow the direction of the beams.
The Great Road, uncovered by them as they walk Southeast along the path of the beam, is built in the direction of of the beam which leads them to the city of Lud.
The first Beam to fall was the Eagle-Lion beam, which ran straight through Gilead. Roland Deschain and his fellow Gunslingers, alongside Marten Broadcloak, witness the city crumble from a Beam-Quake. The remaining Beams then visit Sheemie in a dream to inform him the Beam has broken and that the rest of them are all weakening.
Roland Deschain and his second Ka-tet encounter the cyborg bear guardian in the great western forest near the Western Sea. The cyborg is the bear Shardik, tasked by the Great Old Ones to guard their synthetic beams. Thousands of years later, the bear has fallen victim to millions of larvae that have infested its brain. Sensing Roland and his ka-tet to be trespassers in his woods, he proceeds to attack them. After defeating the bear, they follow the backtrail of the bear to his residence. There, they find the beginning of the beam of bear and turtle and start on their direct route to the Dark Tower.
The Crimson King seeks to topple the tower by destroying the beams. He does, in fact, destroy four of them. When the ka-tet stops the Breakers at Blue Heaven, the Bear-Turtle beam starts to recover, and it is implied that it will heal, and the destroyed beams will start to rebuild themselves. Likewise, it is implied that this means that the worlds that have "moved on" may come back to order and life. While it is impossible to instantly restore the technology of the Old Ones, it can be hoped that a peaceful civilization will again be possible.
These are the Twelve Guardians' statues from Our Lady Of The Rose.