A palace was a structure used as a residence for royalty or nobility.
In the 2250s, Captain Christopher Pike was captured by Dindamorii rebels and held in the governor's palace on Dindamor. He was rescued by Spock and J. Mia Colt. (TOS novel: Legacy)
In 2268, Corben once called the dwelling place of the Inquisitor as a "palace", although it was referred to as a "castle" in every other instance. (TOS comic: "Day of the Inquisitors")
The Nagal Residence was described as a palace. (DS9 novel: The 34th Rule)