The building of the "Chicago Alliance headquarters" is located on the property where Harry's old boarding house used to be. The name on the plaque on the front of the building is the Brighter Future Society which was organized and funded by Baron John Marcone. It cost a fortune to build.
The building is about four stories high and looks like a small castle. It was made of stone and covered more of the property than the old boarding house did. The narrow windows were about nine inches wide—and deep—with bars on the inside and outside. There was blocky crenellation along the roof's edge. Gargoyles were perched along each row starting on the second floor and each row above to the top, making it a fortress.
It has a threshold because people live there. Paranetters stay there when they need a safe place to sleep. Also, visiting Paranetters and Venatori stay there when meeting with the Alliance. It functions as a supernatural flophouse, armory, jail and day care. It's a place to keep children safe from the Fomor while Paranetter parents are at work. In Butters' words: "People need this place."
In the series
In Ghost Story, Waldo Butters takes Harry Dresden's shade there. Dresden watches Karrin Murphy and Skaldi Skjeldson practice fighting. Afterward, Dresden's ghost and Murphy discuss his murder, rescuing Mortimer Lindquist from the Grey Ghost, Fitz and Aristedes.
In Cold Days, Thomas Raith told Dresden that his sister Lara had partnered with the Brighter Future Society and John Marcone, that it was her idea and that Marcone imported the castle. It's better for both of them to team up against the Fomor. The Fomor have not gotten a solid foothold in Chicago between Marcone's goons, Lara's resources and Murphy's people.
After Ace badly injures Dresden with a bat, and Hook shoots him with nails, Murphy suggests bringing him to the Castle since a lot of his blood was left behind. Thomas notes there was no way he'd let John Marcone's people get his brother's blood either.
In Battle Ground, the estate is a blocky stone house with towers at each corner; lit torches burn on the battlements and armors, both modern and ancient, are propped against the walls. Two sets of cars have been rolled to their sides to provide a double barrier on its main entrance. Messengers on bike move from and to, and Hugin and Munin guard the entrance; their animosity towards Dresden has not diminished from the last time they've met.