Anti-Aircraft Guns are artillery/missile launchers strategically located all over Mainframe to destroy or bring down any enemy vessels that dare fly over the city's airspace.
Except for a couple guns at the Principal Office these guns were unseen during the Web War. During the Viral Wars more anti-air craft guns were seen, and by the time Daemon infected the Net they were present in almost every sector. (Web World Wars) (Daemon Rising)
There are two main kinds of anti-aircraft guns; those that are manned by gunners, and their fully-automated counterparts sometimes referred to as 'Smart Cannons'. Binome operated anti-aircraft guns defend the Principal Office, while automated versions were placed in a variety of unsuspecting concealed locations, like underneath sidewalks, pavements, E-mail boxes and garbage bins.
Smart Cannons seem to be fully automated and require no Binomes or Sprites to control them. The guns are very accurate, rarely missiong their targets.
Together with the Firewall, these weapons played a large role in the Viral Wars. Trapped behind the inverted firewall, Megabyte relied on Hexadecimal's power to penetrate the Firewall allowing his armies to spread into the city. He knew perfectly well they would encounter resistance so gambled on winning through sheer numbers by sending several dozen legions under Lieutenant Chauncy, in an all-out attack upon the Principal Office. The anti-air craft guns proved far too much for the ABCs to handle, as they were being shot out of the sky from all directions. Blast debris from a nearby carrier ricocheted past Megabyte, causing Hexadecimal to laugh hysterically at her brother's failure. (Game Over)
However, after the Firewall was disabled the anti-air craft guns simply weren't enough to stop Megabyte from eventually seizing the Principal Office. Hexadecimal's Nulls managed to reduce the vast fleet of ABCs from several hundred to just 39 carriers. Even thought the Principal Office guns fired on the remaining ABC fleet it is unknown if they managed to shoot down any enemy craft. (Megaframe)
The anit-air craft guns were used during the Net War when Daemon's Guardian Armada managed to gain access to Mainframe. The guns shot down the Guardian ships that managed to get into the system, but the surviving Guardians were able to open the system for the rest of their fleet. Bob wished to avoid deleting any of the Guardians and placed himself between the invading armada and Mainframe's long range guns. This prompted Dot to launch their CPU fleet to provide defense for the city. (Daemon Rising)