The Underground Fortress was the military command center on the planet Aquilae.
Surface access to the fortress was made through a secret door that slid open in a rock face, located in a narrow canyon.
It was destroyed by atomic warheads set off by the Aquilaeans themselves, to prevent the Galactic Empire from gaining their genetic and biological secrets. All inside were killed, including Queen Breha. Leia, her brothers and General Luke Skywalker, who were outside the fortress, escaped to Gordon.
Behind the scenes
The destruction of the fortress, though here a suicidal defense, was rewritten into the obliteration of Ogana Major in the second draft, and later became the basis for the fate of Alderaan.