User blog comment:RandomGuy96/So how do planetary defense cannons work?/@comment-184.89.190.70-20130204203232

Those cannons are not for main defense. They are likely supported by fields to protected them. Given they are stationary their reactors can be buried and have no limitations on weight. Simply aligning the discs within the cannon enables the cannon to have a field of view rather than a straight line shot.

From any perspective they are very handy. Easy to protect, hard to get rid of, and are not to suppose to be the main assault force. A determined force would go right through them, but then you wouldn't waste such resources on a place with such low defenses, and if you do gather enough forces you can't hide them as easily.

Like ancient ranged units putting wooden pikes at the begining of a battle. They are not exactly perfect, and they are defensive in nature. Their field of "view" is only towards the enemy, but them alone can obstruct or deprive an enemy from an area. ALthough with sufficient force it would take no trouble.

In short terms... which I guess I should have started with they are meant to buy time, or stall small attackers, a big enough fleet would not have a problem sending a ground force to attack it, but then a large enough fleet would be met in kind.

I admit however the MAC guns of Halo and the ones in ME universe are two opposite ones. But I guess one trades protection for flexibility.