(covers information from several alternate timelines)
A palace was a name for a ornate and lavish residence, typically one home to some kind of dignitary.
During the 2250s, in the mirror universe, the Terran Empire had a mobile palace, located on the Starfleet flagship ISS Charon, for the empire's emperor. In 2257, this palace was destroyed, along with the rest of the starship, by the USS Discovery. (DIS: "Vaulting Ambition", "What's Past Is Prologue")
According to technical data prepared for the San Diego Comic Con in 2018, the Charon palace, located on the upper decks of the starship, had a throne room with a trapdoor. The emperor's quarters were located many decks down from the palace, close to the mycelial 'sun'.