Spire was one of two prison Ages written by Atrus to protect his family and library of Books from any nefarious person who might find his lost Myst Linking Book. It became Sirrus's prison when he and his brother unwittingly fell for their father's trap.
Spire is a dark and cold world unlike any other that Atrus has written. Instead of a planet, the ruin-covered mountains — which Sirrus refers to as "palaces" — are floating chunks of rock suspended between layers of clouds above a green-yellow star-like object (the nature of Spire's "core" is unknown, and it does not appear to follow any known rules of astrophysics). Sirrus speculated that the powerful diamagnetic properties of the green crystal which permeates the palaces is what enables them to remain in such a stable position. There is no life in this Age besides some meager plants, and Atrus states in his journal that no civilization ever set foot on its rocks. Instead, he asserts that the natural forces of wind and erosion sculpted the ruins from the bare stone by chance. Others have suggested that perhaps Atrus was wrong in his assumptions, and that once, many millennia ago, Spire was a whole planet with a thriving civilization that was ravaged and destroyed.
In the 20 years that Sirrus spent on Spire, he built a number of contraptions that allowed him to traverse the main palace more easily, as well as reach a nearby palace with more direct access to the space beneath the lower cloud deck. In a cavern at the base of the main palace, he also built a massive organ, which utilizes the resonant behavior of the Age's crystals and channels the sound up through several geometrically-shaped channels. The organ cavern was later redesigned to serve as a bomb-testing lab after Atrus and Catherine wrote a secure linking chamber into the Age to reconnect with their estranged son, and Sirrus saw a way to escape his prison.
Sirrus was enchanted by the potential for Spire to host a large and easily-manipulated population that he could use for his own ends. Ignoring the warnings his father had given him, he linked to the Age after he and Achenar had waylaid their parents in K'veer. For 20 years Sirrus tried to escape the world by finding the Myst Linking Book that Atrus first used to leave — he assumed Atrus dropped it over the edge, and so climbed down the rock structures, building amazing machines and wiring strange electronics to achieve his goal, only to find that there was no ground to get to. Along the way, Sirrus discovered the source of the strange harmonic sounds that Atrus had remarked on when first visiting the Age: strange crystals which discharge static electricity with a particular resonance. By experimenting with these crystals, Sirrus was able to manufacture some which were capable of disrupting the molecular cohesion of nara — a nearly impregnable synthetic stone — which was also used for the linking chamber Atrus wrote into the Age.