From The TV IV
The barracks were built by the DHARMA Initiative as living quarters for DHARMA personnel on the island. Presumably most, if not all, DHARMA Initiative station personnel were to travel to the barracks when off-duty, but only The Pearl's orientation film specifically states that The Pearl's personnel were to take the Pala Ferry to the barracks after their shift was complete. An orientation film for new DHARMA personnel arriving to the island stated that researchers working at The Hydra could also take the Pala Ferry to get their from the barracks.
The Others were in control of the barracks, using the facility as their home. They took control after they launched a gas attack on the facility, killing everyone there and dumping their bodies in a mass grave. After Kate Austen, Sayid Jarrah and John Locke found the barracks, the Others abandoned the facility to prevent the Oceanic survivors from finding them again.
The barracks are a collection of houses and dormitories with water and power. The facility is within walking distance of The Flame, and electrical and data cabling run from The Flame to the barracks. Footpaths run throughout the barracks, and the Others used bicycles as a faster means of travel than walking. There appears to be no visible roads connected to the barracks, but supply runs were once made from the barracks to other stations like The Pearl and The Flame. Presumably these were dirt roads and have been reclaimed by the jungle after years of disuse. Two subterranean passages are present, about twenty feet underneath the barracks.
The barracks are most likely the question mark on the blast door of The Swan. Due to the security system surrounding the facility, the personnel inside the Swan (namely Kelvin Inman and Radzinsky) would find it difficult to cross over the security system and discover the barracks. As the barracks are not visible from outside the security system, whatever was inside the system was unknown and marked with a question mark.
The houses the barracks consist of appear to have all the conveniences of modern living, including electricity and plumbing. Modern appliances such as ovens, refrigerators, CD players and lamps are present. The inside of Juliet Burke's home was very comfortably furnished with modern furniture, drapes and artwork. Benjamin Linus' home was also decorated with artwork and other personal objects. After his operation, Ben was confined to a wheelchair and the bedroom in his home contained a motorized bed to assist him. While Jack Shephard was a guest of the Others at the barracks, he had his own house that contained a piano. However, this house was outfitted with CCTV cameras to keep an eye on Jack.
The barracks also consisted of a community building with sofas and a coffee table, pinball machines, pool table, foosball table, a fire place and numerous board games, including Monopoly, backgammon and Mouse Trap. Even though this seems to be a rather innocuous building, Tom motioned to Jack that it was bugged with listening devices.
Sonar Security System
A high-frequency sonar security system surrounds the barracks, consisting of a series of pylons. These pylons encircle the entire barracks and house two sensors on either side of each pylon, and these sensors create an invisible plane between each and every pylon. Should the plane be broken by an object, a sonic weapon is activated which causes cerebral hemorrhaging in human victims. Mikhail Bakunin claimed the system was no longer active, but was severely injured by the system when Locke volunteered Mikhail to test the claim by tossing him between two pylons in "Par Avion". He would have been killed had the system not been set to a non-lethal level. While inducing cerebral hemorrhaging in humans does act as a deterrent against trespassers, it would appear this is only a secondary benefit. The main purpose of the sonar fence is to keep the monster from entering the barracks. The sonic weapons the fence employs prevent the monster from passing through the pylons, keeping the barracks and all those inside safe from the monster.
Making it past the system is not a particular easy task. Forming a plan to travel across the security system, Sayid, Locke, Kate and Danielle Rousseau cut down a tree and propped it up against the top of one of the pylons. Being extremely careful so as not to break the plane on either side of the pylon, the group were able to safely traverse the security system by going over the pylon through the gap in the sensors. The security system can, however, be deactivated when inside the barrier. A green box is mounted on the each side of some of the pylons. Entering the correct code into a keypad on the face of the box unlocks the faceplate so it can be flipped open and allow the user access to the power controls, as demonstrated by Juliet in "Left Behind" and a young Ben in "The Man Behind the Curtain". Powering up the security system is nearly instantaneous.