Atrus wrote Spire as a Prison Age to protect his library on Myst. He wrote it having in mind a soaring, rock-and-crystal spire rising out of dense clouds. To a greedy explorer, it would seem like the watchtower of some gem-studded castle.
On his first and sole foray to the Age, Atrus linked to the tower formation and then descended on rough stone steps towards an ocean of clouds and an empty terrace area. He walked through massive stony archways searching for a view beneath the clouds. The mysterious harmonic sounds made him feel that he was being followed by the ghosts of an ancient civilization. He found floral caverns and was reassured that the Age can support human life.
He was sad that he had no time to determine the source of the sounds, or study the Age to learn more about the Art. He had to link back to Myst, holding the linking book over the clouds, letting it drop to the void. Afterwards he investigated Haven. That evening he wrote two linking books, one for Haven, and one for Spire, and placed them on display in the library. He warned Catherine, Sirrus and Achenar to stay away from them.
Neglecting his father's warning, Sirrus was lured by the castle formations, and linked to Spire.
There were many rocks in Spire with odd properties that Sirrus used to develop electronics, vehicles, and weapons while imprisoned. Catherine wrote in a nara dome at the top of the spire so the family could easily visit Sirrus without his release, but Sirrus used one of his weapons (a nara bomb) to crack this open and escape to Tomahna.
The Age is made up of large mineral-rich rock and crystal floating islands along several layers of clouds, with spires emerging from them. The Age is characterized by stormy weather. No civilizations ever inhabited this Age although the land was purposefully made to be shaped with natural formations that resemble stairways, gigantic detailed columns and perfectly dimensioned arches that have sharp angles and even designs carved in them. In reality, all these were formed by the corrosive power of wind.
Oddly-shaped "windpipes" lead inside the mass of the rocks, ending up to magnificent crystal caverns.
In every place of the Age, crystals created beautiful harmonic sounds.
From the link site only two land masses can be seen above the clouds: the spire, with steps leading to a an empty terrace area and then floral gardens. Sirrus made his living quarters in the garden between the two cloud layers. Flashes of light move through the second layer.
sirrus observed a "floating palace", which is a far smaller area. Sirrus had to install a rock-ship to traverse the gap and reach the "palace".
The only form of life are some scarce green plants, not scrumptious but they were nutritious enough so that Sirrus cultivated them in a garden, and survived.
All of this is hovering far above the surface of the planet, which is a luminescent green sea.
While the land of Spire is mostly solid rock, it is also mineral- and crystal-rich, with some of these crystals demonstrating unique properties. These allowed Sirrus to harness large amounts of power. In addition to the rock and different metals, there are three main crystal types found naturally on Spire. Nara was also written into the Age when Catherine created the visiting dome.
- White crystals: This crystal is mined by Sirrus in the lower caverns. Its inner matrix conducts and stores large amounts of electric energy, giving off a faint light when charged, and humming when this energy is released (i.e. by touching a grounded object). Sirrus managed to harness static electricity from the Spire's stormy environment by charging them, and thus store and generate large amounts of electricity.
- Green crystals: This crystal naturally floats due to strong diamagnetic properties. Unless it is restrained it will hover far above the ground at approximately the same height as all others of its kind. Larger boulders fly higher. This floating ability was used by Sirrus to construct a rock-ship to sail to other rock islands. He combined the magnetic attractive force with the smaller magnetic fields of the lightning conductors and thus was able to lower and steer a boulder.
- Blue crystals: No known special attributes.