Activated by ten emitters located throughout the city, the shield is used to protect the city from everything trying to harm it, including weapons fire, ocean pressure, and even solar and nuclear radiation and asteroids. It is also a critical part of the city whenever its Stardrive is used, since the shield prevents air from leaving the city, thus acting as a sort of outer hull. (SGA: "Rising", "The Siege, Part 3", "The Return, Part 2", "Adrift", "The Last Man")
The shield requires massive amounts of energy to be activated, which a Zero Point Module usually provides; however, the energy of several lightning strikes will provide enough energy to power the shield for some time. When Atlantis submerges under the ocean, one ZPM can power the shield for at least 3.3 thousand years holding back the colossal amount of water above it. (SGA: "Rising", "The Storm", "The Eye")
The shield of Atlantis is unique in that almost nothing can penetrate the shield as long as there is enough power to sustain it (unlike starships, which deplete under external forces and slowly lose integrity no matter how much power is available). If there is no pressure on the shield, it can remain active almost indefinitely. However, even mid-level energy weapons (like those on Wraith ships) will rather quickly deplete the power to the city shield under sustained bombardment, when powered with only one ZPM. It is unknown how the city shield would fare under sustained fire from extremely high-level energy weapons like the Asgard plasma beam weapons, the Ori energy beam weapon or repeated strikes from Drone weapons.
Depending on the power of the ships facing it, Atlantis' shield will fare differently in space battle. During the Battle of the Super-hive, while powered by three fully-charged ZPMs, Atlantis' shield quickly started to weaken under the fire of the super-hive, reaching 70% with only a few hits. This was further exacerbated by the fact that Atlantis was pushed into the Earth's atmosphere with the friction further draining the shield. In contrast, while battling Queen Death's fleet of hive ships and cruisers and powered by one fully-charged ZPM and only slightly-charged ZPM, the shield held against tremendous strain. At only one point did the shield drop below 70% due to a powerful explosion at close-range and it quickly recharged back over 70%. At the end of the battle, the shield was still holding at around 70% despite the tremendous strain it had been put under. (SGA: "Enemy at the Gate", "Inheritors")
While the shield is typically used to protect only the city and the immediately surrounding area, it can be expanded to cover a significant portion of an average-sized planet or even shrunken to encompass individual objects inside the city itself. Using the shield in this way, however, depletes far more energy than what would typically be used up and reduces the shield's effectiveness. The Ancients used the expansion ability to protect the ecosystem of Lantea from the periodic Coronal mass ejections of the nearby sun. The Atlantis expedition shrunk the shield to surround the Stargate when it exploded because of the effects of the Attero device; however, the strength of the explosion was greatly amplified when the shield compressed the explosion, and the massive amount of energy (nearly 12 Naquadah-enhanced Nuclear warheads) inside the shield caused the emitters to fry out and the shield to collapse, releasing the remaining energy and damaging a large portion of Stargate Operations. (SGA: "Echoes", "First Contact")
When landing the city on a planet, the shield is required to protect it from reentry. Too steep of an angle will cause the shield to attenuate under the stress and collapse. However, the shield doesn't seem to be at a particularly high level to survive reentry as it was on the verge of collapse when Atlantis was forced to reenter Earth's atmosphere during the Battle of the Super-hive though it seemed to drain what was left of the shield. When Atlantis was landed on the frozen planet, Sheppard took the city in on a shallow angle at first which put stress on the shield but also burned off a great deal of speed. Doctor Carson Beckett speculated that this was how the city was supposed to be landed. (SGA: "Lifeline", "Enemy at the Gate", "Legacy: Homecoming")
Over time, the Atlantis expedition has discovered alternative uses for the shield of Atlantis. The first such time was during the Wraith attack on Atlantis. In order to trick the Wraith into believing Atlantis was destroyed, the expedition converted the shield into a massive cloak, turning the whole city invisible to both visual and mechanical means of observation. When the shield is used in this manner, the emitters draw much less power than normal, allowing the cloak to remain active for far longer than the shield could when under no stress. The downside to using the shield in this way was that the cloak is simply the shield operating on another frequency, meaning the shield is replaced entirely and the city is vulnerable to attack if an enemy is not fooled, as it takes time to reconfigure the cloak back to a shield. The Atlantis expedition has since been able to reduce this time to only a few seconds. (SGA: "The Siege, Part 3", "Enemy at the Gate")
After the Asurans took over Atlantis, Dr. Rodney McKay developed a means to convert the entire shield into a massive Anti-Replicator field: by inserting the primary Control crystal of an Anti-Replicator gun into all ten shield emitters, every Replicator in the city would fall apart when the shield was activated. (SGA: "The Return, Part 2")
Despite being impenetrable to virtually any means of attack while power is available, the shield of Atlantis is usually configured to allow any Ancient vessel, particularly Puddle Jumpers, to pass through it by generating an unknown form of electromagnetic field. Unfortunately, any sufficiently advanced race with access to an Ancient ship equipped with this mechanism and a reasonable understanding of Ancient technology can duplicate this bypass effect and fly an object straight through the shield. After a mysterious ship stole part of the Attero device and kidnapped both Dr. Rodney McKay and Dr. Daniel Jackson, the shield was reconfigured to prevent this from ever happening again. It is unknown if Jumpers will be able to pass through the shield in the future if needed, or if the shield must be temporarily reset whenever such a situation arises. (SGA: "First Contact", "The Lost Tribe")
Another weakness of the shield is that it does not encompass the whole city immediately when activated, instead rising from the bottom up and from the edges of the city inward, allowing a brief window for attacks to damage the city by passing through the hole in the shield before it completely closes. This weakness was discovered when the satellite launched by the Asurans managed to damage the city while it was escaping from the planet, causing it to drop out of hyperspace prematurely. It is possible that the shield's slow formation, however, was due to there being only one Zero Point Module. With the significant power drain coming from the Stardrive, less energy would have been available to the shield. In situations where there has been no other drain on power, such as during the Lantean storm, the shield was able to fully form in around a second. (SGA: "The Eye", "First Strike")
A much less obvious weakness is that, if the city is using its Stardrive, particiularly intense weapons fire will exert enough force to actually alter the city's trajectory to an extent that the inertial dampeners cannot properly compensate. When this happens, damage to systems that are not secured as well as on normal starships is sustained, as the city was not designed for space combat. However, this is partially due to the fact that Atlantis was facing a super-hive at the time and was in close orbit to Earth. When further away and with hive ships with less power, the shield held up very well and was shown to not drop very far during the battle. At one point, it was mentioned that a particularly close pass that damaged the East Pier maneuvering engine dropped the shield to around sixty percent but it quickly recharged to eighty-nine percent and was climbing. (SGA: "Enemy at the Gate", "Inheritors")
- In an alternate timeline where Atlantis was used by the Wraith to reach Earth, the Wraith had control of Atlantis' shield, using it to prevent enemy attack, particularly by the Goa'uld System Lord Hecate. When the Wraith Commander Sting approached the city in 2098 as part of a joint mission with Colonel Jack O'Neill and Major Samantha Carter, he had to deceive a Wraith inside the city into lowering the shield long enough for his Wraith Dart to pass. The shield later easily held up to weapons fire from Hecate's Ha'tak, fired by Carter to entice O'Neill into using Atlantis to destroy the ship. (Stargate SG-1: Apocalypse)
- In an Alternate reality where the expedition arrived in Atlantis but then evacuated it en masse due to the power depletion, SG-1 were able to release the city from its position on the ocean manually when they returned to the city to find a way back to their world, subsequently directing the expedition to a new source of power in the form of a dimensional teleporter devised by Janus (Stargate SG-1: Ouroboros).
- In an alternate timeline where the Atlantis expedition arrived with the city's ZPMs all near-depletion and with no failsafe mechanism in place, Atlantis' shield began to fail much faster and more all at once than failing in sections to conserve power. When the shield failed completely, most of the expedition drowned as the city flooded. (SGA: "Before I Sleep")