My Citadel has:
Modules: 1 Shipyard, 5 Gun Batteries
Buildings: Target Uplink Computer , Defense Grid Supercomputer (this one is important as it allows for 8 additional platforms), Crew Quarters, Command Center.
Ascension Perk: Eternal Vigilance. This allows for 25% boost to starbase and defense platform damage as well as being able to build 5 additional platforms.
Technology: Titans. This allows you to construct ion cannons as a defense platform.
Defense platforms: (with all perks, tech and building there should be space for 28) 3 ion cannons. The rest is platforms set up with heavy station sections equipped with level 2 Kinetic Artillery.