B'Alda'ar Base was a planet-based facility located inside the Klingon Empire.
Kor and Curzon Dax met at B'Alda'ar Base in 2344, where they discussed Chancellor Kravokh's obsession with Ch'gran. (ST - The Lost Era novel: The Art of the Impossible)
Kor again visited B'Alda'ar Base in early 2373, where Jadzia Dax and Worf appealed for his help in informing the Klingon High Council of the attempted assassination of Captains Jean-Luc Picard and Benjamin Sisko by agents claiming to act in Chancellor Gowron's name. (TNG - Slings and Arrows eBook: Enterprises of Great Pitch and Moment)
The Kreel attacked B'Alda'ar Base sometime prior to 2376, while Doctor Valatra was present. Valatra performed a surgery there similar to one taught at Starfleet Medical Academy, though not common in Klingon medicine. (KE novel: A Burning House)