The phrase "underground fortress" refers to a hidden and well-protected structure or building that is built below the ground level. It suggests a secure and secretive place that is designed to defend against external threats. Full definition
Fortunately, an impenetrable underground fortress will help keep your family sheltered from the desolate post-apocalyptic wasteland above. (gamemakerblog.com)
Following a terrible car accident, a young woman (Mary Elizabeth Winstead) awakens in the underground fortress of a doomsday prepper (John Goodman). (shepherdexpress.com)
It shows some of the incredible planning and preparation that went into building what is basically the film's entire set — the underground fortress of District 13. (slashfilm.com)