Gahreesen is an Age that was formerly owned by the Guild of Maintainers. Its name is an amalgam of three D'ni words: "gah-ro" (great/mighty), "ah-ree-uh" (to protect), and "sen-ah-ren" (building/structure). Thus, the Age's name is, quite literally, "mighty building for protection". RAWA more aptly translates the phrase to "Fortress" but admits that the DRC's choice of the word "Garrison" (by which the Age is sometimes referred to) is understandable and still fairly appropriate.
Gahreesen is a lush, jungle-like Age. Outside of the guild's structures, the terrain is verdant, with small patches of trees sprouting up from time to time. The Maintainers constructed their facilities in a valley, close to several waterfalls that cascade into a nearby lake. Birdsong echoes throughout the valley, although no birds have been directly observed in the Age.
The buildings in Gahreesen were designed to constantly rotate, which allowed no linking Books to be written from within the complex. It also meant that any books which linked to the Age needed to be on special timers so that the person linking in would find themselves where they expected to, and not in another room or even inside a wall.
The first building, referred to as "The Well", is mostly embedded in the hillside, and its rotation is driven by a waterfall. The building takes its name from the link-in point: a high-walled central chamber which is open to the air above it. A two-story ring of rooms surrounds this chamber, which are themselves encircled by a hallway that runs around the outermost edge of the building. These rooms were used for a variety of purposes, including screening visitors and distributing KIs to authorized individuals. Two elevators connect the interior floors to the roof—one takes visitors up, and the other back down. On the second floor, a massive gear connects to the stationary outer wall. When the gear is engaged, it provides power to the facility's doors and elevators.
The second building is a training facility. This Training Center is built around a massive interior arena called the Wall, where trainers could manufacture harsh conditions such as heat or toxic air. Teams of guildsmen used this Wall for training in environmental protection suits. The rest of the Training Center is divided into two identical halves, each with a staging room, a suit storage room, and a conference room which looks out onto the Wall. The roof of the Training Center served a very different purpose: a prison. A ring of small cells are buried in the outer wall of the structure. A metal ladder in the drainage chambers below the cells provides access to the rooftop. There is a central chamber here filled with machinery and electronics, though its purpose is currently unknown.
The Training Center's lower levels are only accessible by jumping from the Well to a small spire of rock, and then jumping from there to one of six long bridges which lead into the Training Center. While this was originally developed as a rather simple security bottleneck, centuries of erosion has resulted in the jump being rather more perilous now than it used to be.
The Guild of Maintainers used Gahreesen as a special projects facility, where members could develop and test new technologies in a highly secured environment. The Age was written in 8500 DE, and was used up until the fall of D'ni in 9400 DE.
The Well is where the KI was developed, and distribution of the technology was built into the facility's lower floor. By the time D'ni fell, it had been partially converted for public access when the KIs became a more widespread device. The D'ni re-enforced the building's built-in security with guards and Linking Book-detecting beetles. Visitors arriving in the well were escorted through a security checkpoint to the KI distribution room, then returned to D'ni after leaving through a separate checkpoint.
The Maintainers used the Wall inside the Training Center to train and test new maintainers. Two teams of guildsmen were pitted against each other in a time trial, and tasked with climbing the Wall while enduring harsh manufactured conditions and avoiding barricades placed on the structure by the opposing team. The upper levels of the building were used as a prison, and prisoners were stored in cells using a complex timer tied to a Linking Book to ensure they went to the proper location. Notably, the bahro stone linking to the prison does not need this mechanism; explorers are always deposited in the same cell.
In 2002, the DRC restored enough of the Well to get KI distribution back online. Visitors to D'ni were directed to Gahreesen to obtain one. In late 2003, a journey door appeared in the cliff face outside the Well as part of Yeesha's Journey. Over the years, the DRC has also intermittently granted players access to the Wall, though it is currently offline.
Additional areas of Gahreesen – Gahreesen II and Gahreesen III – were scheduled for restoration, but never released. Gahreesen II was stuck in phase 4 of the process throughout 2007 and 2008, when the DRC again abandoned their efforts entirely. Restoration of Gahreesen III was suspended on August 16, 2006, based on reports from a restoration engineer describing dangerous creatures in the area.