The link-in point for the Age was at the edge of a large, steep-sided, circular depression containing a lake. At the center of the lake was a small island on which stood a massive tree. The Moiety used this tree as their home, constructing a large "hive" structure that was interwoven into its branches. The link-in point was also well-guarded and contained a small cave, where an effigy of Gehn stood holding the link back to Riven.
This was one of many Book that had been written by Gehn in his attempts to re-develop functioning Descriptive Books during his imprisonment on the Age of Riven. He had deemed the Book a failure, and so discarded it in his lab. The Moiety were able to obtain it, and bring it to Catherine, who was herself trapped on Riven at the time. She made a number of modifications to the Book, and it ultimately became an off-world refuge for the Moiety during their struggle against Gehn.
Independence from Gehn
One of the most important changes that Catherine made was the inclusion of a special type of geode. These geodes, when sliced, could stabilize Gehn's defective Books, allowing them to be used without requiring power from his firemarble domes. Catherine and her rebel allies managed to sneak into Gehn's firemarble domes to power the link for its first use, allowing Catherine to travel to Tay, obtain one of the geodes, and use it to stabilize her link back to Riven.
After Riven's collapse
When Riven its final collapse, Catherine and the Moiety transported the rest of the Rivenese people to Tay. They lived peacefully there, and Catherine remained in contact with them for many years after. She also seemed to be in the habit of bringing Yeesha there to visit as well, as she does so following the unfortunate events that transpired during Myst IV: Revelation.