Outposts and Stations were the tools you used to bring the universe under your control. It is time to set sights on grander ambitions. Citadels are massive player-created cities in space, dwarfing almost everything the Empires have ever constructed. These monuments to the might and power of capsuleers make excellent trade hubs, hangar/residential quarters, and defensive stations.
Medium, Large, and XL Citadels - the Astrahus, Fortizar, and Keepstar respectively - the largest of which is massive enough to offer super-capital ship docking bays.
Module Slots allow for weaponry, electronic warfare and rigging components to be installed.
Modular construction to add markets, clone units, compression systems and other facilities.
Ship Tethering provides security for friendly ships close to the Citadel, granting safe passage in and out of the structure.
Unlimited Personal and Corporation Hangars provide residence for even the largest of Alliances.
Multiple Undock Points, including separate capital (L Citadel/Fortizar) and super-capital (XL Citadel/Keepstar) undock areas, prevent traffic at busy hubs.
Each citadel has a weekly maintenance period where recalibration is handled. The larger the citadel, the larger the maintenance window. It is during this time that citadels are vulnerable to attack.
The cost to construct a citadel can range from 700 million to over 120 billion depending on the size of the citadels and its amenities. A trivial price for a fortified city among the stars.
Read this dev blog on structure fitting