User blog comment:Braveangel/About planetary defenses/@comment-217683-20100423191838

Because, as I pointed out above, dreadnoughts are still combat-capable at short ranges, as demonstrated by the fact that, for example, Kilimanjaro class dreadnoughts have broadside armaments (76 guns on each broadside). Hunter becomes the hunted? I think not. At this point, all that happens is that the dreadnought stops fighting like a 'dreadnought-style battleship' (note there is no such thing as a 'modern battleship', as they haven't been built since the '50s and no navies use them today. And the most recent battleships used secondary weapons of the same caliber as the main batteries of heavy cruisers- hardly pretty lame) and starts fighting like a ship of the line.

So again, given that the Citadel had 5 layers of defenses as it was (the nebula, the Citadel Fleet, the expected rapid reinforcements, C-Sec, and the closing of the Station to use it's impenetrable shell to defend itself) it seems pretty obvious why they didn't shell out for further defenses. They didn't feel they needed them, which does also reflect on the mindset of the Council themselves.

Finally, an interesting side fact- you mention the BSG miniseries (aka the 'prologue') in which the Cylons (as opposed to Ceylon, which is an antiquated name for Sri Lanka) must disable the Colonial defenses. And what do the Colonial defenses consist of in their entirety? A fleet. In other words, pretty much the same as the Citadel. Does that mean it's a plot hole? :P