The automated cannons are fixed surface-to-air weapons, used to fire at flying targets, usually gargoyles. They are sometimes used by wealthy individuals as defensive installations to protect their buildings. David Xanatos, Macbeth and Taro have all used them.
When Goliath, Hudson, Brooklyn, and Lexington charged Macbeth's Mansion to rescue Broadway, Banquo and Fleance manned the cannons. The cannons held the gargoyles at bay for a time, but it caused significant damage to the mansion. Goliath was able to get close enough to pull Fleance from her cannon. Hudson rammed himself into Banquo's cannon, causing it to fire down and destroy itself. This was the only time a cannon ever had an operator.
When Goliath thought Macbeth was the cause of Hudson being trapped in his stone sleep at night, he and Lexington went to Macbeth's mansion in search of a cure. The cannons had been repaired and this time were totally automated, programmed to fire at moving targets. Goliath and Lexington each got on a cannon, causing them to fire at one another and destroy themselves.
Thinking that Xanatos was behind Goliath, Elisa, and Bronx's disappearance, Talon and the trio stormed the Eyrie Building. Seeing that they were en route, Xanatos and Owen decided to test the new security system. Two cannons that had been installed in Castle Wyvern's towers emerged and fired on the gargoyles and their mutate ally, but instead caused significant damage to the castle. Broadway was able to get on one of the cannons and bend its barrels up causing it to fire and destroy the other cannon. The cannon, not being able to fire, swelled and exploded.
When the Ishimura Clan tried to leave his theme park, Taro activated two automated cannons to keep the gargoyles in. These cannons were different in that the blasts they fired only stunned the gargoyles. Hiroshi knocked out one of Taro's ninjas in the control room and pushed a button to deactivate the cannons.
As weapons, the cannons were largely ineffectual against their intended targets and often caused much damage to the place they were installed in to protect. It is unknown where and how Macbeth, Xanatos, Taro acquired them.