I like the idea that the castle is a dimensional projection of a creature on another plane (kind of like the mice in HHGTTG).
Pick suitable nasty BBEG, have them reside on Plane X, have them somehow get stretched or warped into the Material Plane where they appear as a castle. Then take their stat block and enhance any actions and stuff they have and apply it to things inside the castle.
Say the creature is a red dragon (adult), they have a bunch of fire related attacks, and you can use their Legendary Actions as they are their own lair hehe, so there might be traps that shoot fireballs out, or fire elementals come out of a fireplace, etc etc etc... And any movement applies to any minions you put inside, like Pounce for the red dragon can be applied to a group of red dragon wyrmlings that travel the hallways as some sort of guard.