Is mansion and palace the same?

As nouns the difference between palace and mansion

is that palace is official residence of a head of state or other dignitary, especially in a monarchical or imperial governmental system while mansion is (senseid) a large house or building, usually built for the wealthy.

What qualifies a castle to be a castle?

The word 'castle' derives from the old English word 'castel', which meant village. ... And now the Oxford English Dictionary defines a castle as 'a large building, typically of the medieval period, fortified against attack with thick walls, battlements, towers, and often a moat'.