The Ba'ul stronghold was a massive Ba'ul structure located beneath the surface of a lake on the planet Kaminar. It measured at least fifty kilometers across, and controlled the network of Ba'ul pylons across the planet.
In 2257, Commander Saru and his sister Siranna were taken to this facility after the Ba'ul detected Saru's return to Kaminar. Since both had learned the truth about the vahar'ai, the Ba'ul had planned to analyze both of them, and then neutralize them. Saru broke free before this could occur, however, and in coordination with the USS Discovery crew, he used the Ba'ul technology to trigger vahar'ai in all Kelpiens simultaneously. Because of this, the Ba'ul raised the facility to the surface of the lake and prepared to use it to exterminate the Kelpiens, but the Red Angel prevented this from occurring by using an electromagnetic pulse to disable the facility and destroy the pylons before disappearing again. (DIS: "The Sound of Thunder")
The interior of this facility was a re-dress of the USS Discovery's transporter room.