User blog comment:Braveangel/About planetary defenses/@comment-217683-20100423055030

Dreadnoughts are useless, eh? Then why was the Destiny Ascension even there? Apparently because dreadnoughts aren't useless at the ranges in question. Remember, the "knife-fight" ranges you mention are (canonically) ranges of no more than a few DOZEN kilometers. And, as the cinematics show, the Citadel is more than a few dozen km from the relay.

Again, the Citadel is maintained by the keepers, and due to its design, the Citadel races aren't able to access its inner workings, which would likely prevent them from tapping into its power grid to install turrets, and the keepers are also in the habit of (seemingly) randomly moving around or removing things that are added onto (or into) the station after-the-fact by the aforementioned Citadel races.

As for plot hole or not, we know that in-game, there ARE defensive space stations. And there ARE heavily defended space station interiors. The Citadel lacks these. Now, if they were completely absent from the series, maybe a plot hole. But they aren't. They are only absent from the Citadel, which is (speaking in terms of plot) completely consistent with the arrogance, hubris, and general 'we know it all' attitude of the Citadel Council, which seems to imply it isn't a plot hole.

As for Horizon, stating the batarians would hit it sooner or later if it were undefended is speculation, at best. All we know is that the people who went there to avoid having anything to do with the Alliance don't want the Alliance coming to their world and setting up weapons and personnel. Pretty understandable, no? And again, it is pretty important plot-wise for both of them to be undefended, or minimally defended, isn't it? If, as you say, Horizon had GARDIANS installed which made it so that "Collector ship couldn't have landed on the planet", then a big chunk of the ME2 storyline wouldn't have happened. Long story short, we know that the Citadel and Horizon COULD have had stronger defenses. For whatever reason, they didn't. Given that this seems completely consistent with the attitudes of the people living there, it doesn't seem to be a plot hole.