Citadel is EVE Online's largest expansion in years. It went live last week on April 27. If you thought the Titan-class ships were enormous (and they absolutely are), then wait till you get a load of citadels, the "Cities in the Heavens." It takes sizeable corporations of players months to build Titans. I can only imagine how long—and how many in-game resources—it'll require to build one of these cities. Suffice it to say, only the biggest, most highly motivated corps will make one of these happen. There's the Astrahus (medium), Fortizar (large), and Keepstar (XL) citadels.
But, as this video will attest, taking one down will be a battle for the history books, too. Citadels explode real nice.
In addition to citadels, the (free) Citadel expansion makes major changes to capital ship mechanics and warfare, introduces a whole new class of capital ship, brings on new capital PvE encounters, and kicks out a lengthy list of UI upgrades.
Here's a flyby video of a citadel. So many blinky lights. Can't wait to see EVE players put their nose to the grindstone, as only they can do, and get one of these up and operational.