BACKGROUND
Atlantis was once inhabited by a highly advanced alien race who called themselves the Ancients. Or perhaps not so alien: Not only did the Ancients possess a humanoid structure, but they were the first evolution of that form -- human beings came second.

Atlantis was believed to be the Ancients' base of operations. It was originally located on Earth, on the continent that was to become Antarctica. Between five and ten million years ago, under the threat of a deadly plague, the Ancients were forced to leave Earth with the great city (which, when running at full power, is capable of space flight) relocating it to a new planet in the distant Pegasus Galaxy.

Here, the Ancients found more than they had bargained for. Never before had they encountered any species that rivalled their own might -- this came to an end with the awakening of the Wraith, an enemy to which they had inadvertently given birth. Entrenched in a gruelling war which they had no chance of winning, the surviving Ancients submerged their city in their planet's ocean and returned to Earth, where they either ascended or lived out the rest of their days in peace.

We'll get back to them. Let's see what's happening on Earth. At the dawn of the 21st century, the small, remaining outpost was discovered by a group of people who had been using another piece of Ancient technology to explore their own galaxy -- the Stargate. Following this discovery, an international research team, headed by Dr. Elizabeth Weir, was assembled to study the facility and its technology. Through their research, the team was eventually able to determine the coordinates to Atlantis's new home, and in 2004 Weir led an expedition through the Stargate to uncover the lost city.

Upon their arrival, the city's systems began reactivating automatically -- depleting the overall power supply and shrinking the shields that were holding back the ocean. Automated systems raised the city to the surface before the shield could fail, and it now sits as an island of sorts in the middle of the ocean, with a large land mass only 25 minutes travel by Puddle Jumper -- the name given by humans to the small Ancient ships left behind in Atlantis.

THE CITY
Vital to the operation of the city is the Zero Point Module, or ZPM. Atlantis is designed to be powered by three such power modules, however at the time the human expedition arrived, the city's ZPMs were critically depleted. Operations were bolstered naquadah generators brought from Earth, however power remained a difficulty for some time and critical systems such as the shield could not be powered until a new ZPM was provided from Earth.

Atlantis is also in possession of the only Stargate capable of connecting to worlds outside of Pegasus - Earth included. Due to the security risk that this presents, and to prevent capture from the Wraith, self-destruct codes have been given to all officers of the facility. In addition to the self-destruct sequence, a computer virus has been devised to wipe the Ancient database clean, to prevent any possible recovery of information in the event of a defeat.

Transport chambers are situated at various locations around the city. These could best be compared to highly advanced elevators -- they allow fast and easy access to key locations in the city, which include:

Gate Room A massive area that holds the Stargate, and is suitable for large numbers of departing travellers - or a small space vessel, such as a Puddle Jumper. The 'gate itself is situated on a platform in the centre, allowing easy access from any direction. Stairs lead downward from the Stargate, and a staircase directly before the 'gate leads up to the operations and conference rooms. Small terraces overlook the gate room on either side of the second level, and the ceiling retracts to allow access to and from the Puddle Jumper bay.
Operations Centre Positioned across and above the Stargate area, the operations area is the control centre for the entire city. It is equipped with various control stations that allow for the monitoring and maintenance of the base. The dialling system for the Stargate is also located in this area, being the city's inbuilt equivalent of a DHD. From within the control tower personnel can also control the Stargate forcefield, access the city's database of Stargate addresses, manage the city's power resources and city-wide communications systems, and more.
V.R. Room Featuring a large, three-dimensional holographic interface, this room is a store of recorded visual information. The history of the Ancients, as well as their star maps, other sequences may be projected in this room by use of an operation console. This room isn't used often due to the huge amount of power it consumes.
Medical Bay The central medical facility, this room can accommodate multiple sick and wounded personnel. The area is also well-equipped for biological research, making it a useful base for Dr. Beckett's on-going gene therapy and medical research.
Puddle Jumper Bay The spacious, circular Puddle Jumper bay is located directly above the gate room and houses multiple decks of the small Ancient vessels. For 'gate travel, panels on the floor retract allowing a Jumper to be maneuvered into the gate room below and through the Stargate. The roof is also retractable, allowing ships easy entry and exit to and from the city.
Conference Room A basic conference area with a large table, suitable for conducting mission briefings and other meetings. The conference room is located in the control tower, directly across from the operations centre. A series of multi-panel doors open onto a terrace overlooking the Stargate.
Living Quarters Since the Ancients themselves were human, accommodations could not have been more perfect for the members of the expedition from Earth. Some quarters hold several beds, making them suitable for multiple individuals, including families. Other quarters, however, are smaller and suited for a single occupant.
Mess Hall The main dining area in Atlantis, where meals are served and where personnel may eat and relax. With ample seating, this place could feed a small army. In fact, right now it does.
Gym With its expansive floor space, the gym area is suitable for most kinds of physical exercise. It's situated quite far from the main city, near the east pier.
The Brig The city's lockup, where all prisoners are held. Sturdy constructs on their own, the cell walls appear to be reinforced with an energy shield, which holds strong against even the most formidable of prisoners.

There are also multiple scientific laboratories located around Atlantis... and what else, well, who knows? The city itself - which is roughly the size of Manhattan - is largely unexplored, and no doubt many of its secrets still wait to be uncovered by the second evolution of humankind.

Thanks to GateWorld for this information.