User blog comment:RandomGuy96/So how do planetary defense cannons work?/@comment-4973403-20130128220226

Defense cannons are purely defensive weapons. Anti-Spacecraft cannons would be virtually worthless without a supporting fleet in orbit or in the vicinity of the planet. The anti-spacecraft cannon would be for supporting the defending fleet. A more localized defense such as on Horizon more than likely cannot fire into orbit (I believe that they would have to be particle beams as opposed to lasers as lasers are a solid beam), and are more for a local defense against dropships/shuttles (although if a cruiser manages to land on a planet, such as the Collector Cruiser, it will be a sitting duck, although the cannons were not able to disable the ship.) Anti-Spacecraft cannons could also function as large FLAK cannons providing a spread of fire while to discourage an invading fleet from making a direct assault or to breakup an attack, while the defending fleet manuvres around to flank the invading fleet, or is able to withdraw to the far side of the planet in order to regroup under covering fire. On Rannoch, Koris' ship was shot down after absorbing fire meant for the Civilian Fleet (So long Quib Quib). The Quarians rushed in for their attack, relying on surprise, hoping to defeat the Geth fleet before they could organize an effective rally and counterattack; they did not perform recon, so they had no idea where or even if there were any cannons on the surface. It souded like the Geth Fleet was attempting to pin the Quarians against Rannoch allowing the cannons to hit from one side while the fleet hit the other. The destruction of the Geth Dreadnaught caused the Geth to become unfocused allowing the Quarians to escape the trap. If anyone remembers the original Masters of Orion, no matter how many panetside missile bases you had, they were worthless against a strong enough fleet when un-supported.