User blog comment:Braveangel/About planetary defenses/@comment-1293425-20100430052337

Saren's base on Virmire had a system of AA towers, the Citadel had a bunch of turrets Shepard could activate to fend off a geth dropship, and Horizon had the GARDIAN lasers. I don't see a lack of defense turrets in situations that would call for them. Defense space stations are somewhat missing, but Gianna Parasini mentions battlestations in orbit of Noveria, so they do exist. A system of heavy defense turrets for the Citadel would just seem redundant in the presence of the Citadel fleet. How could it succeed where the fleet failed? If anything, they're just better targets, being immobile.It's not like you can put things on the turrets that you can't put on a spaceship. In fact, the recoil inherent to mass accelerators would make such turrets problematic, since they would move with no way to return to their former position.

BTW the Citadel arms' armor layer is estimated to be able to take days of concentrated bombardment with the most advanced weaponry to penetrate it. So this is, in fact, one clam that can take a hell of a pounding.