A castle was a fortified stronghold surrounding a keep. Humans and various alien species have been known to build them, often with incredibly similar designs.
In 2372, Miles O'Brien and Julian Bashir visited the castle of the King of Leinster in a holosuite program of ancient Ireland. O'Brien insulted the King in his keep. (DS9: "Accession")
In 2374, O'Brien toasted the proposed retaking of Deep Space 9, calling it a castle. Bashir added "and planting our flag on its battlements, once again." Jadzia Dax thought they had been spending too much time in the holosuite. (DS9: "Favor the Bold")
In 2375, Elim Garak compared the Cardassian Rebellion's assault on Dominion Headquarters to storming a castle. (DS9: "What You Leave Behind")
One of the stories in The Adventures of Flotter, a children's holodeck program, involved a giant beetle that lived in a castle. (VOY: "Once Upon a Time")
Prior to the making of Star Trek: First Contact, an early story, titled Star Trek: Renaissance, which involved a Borg hive hidden in a castle basement was proposed.