a man's home is his castle

Traditional; the sentiment dates back to Roman times:

quid enim sanctius, quid omni religione munitius, quam domus unusquisque civium?

What more sacred, what more strongly guarded by every holy feeling, than a man's own home?

—Cicero

In English, see Book 4, Chapter 16 of William Blackstone's Commentaries on the Laws of England:

And the law of England has so particular and tender a regard to the immunity of a man's house, that it stiles it his castle, and will never suffer it to be violated with immunity: agreeing herein with the sentiments of ancient Rome, as expressed in the works of Tully; quid enim sanctius, quid omni religione munitius, quam domus unusquisque civium?

a man's home is his castle

1. A proverbial expression of personal privacy and security.

   Synonym: (UK) an Englishman's home is his castle

• castle doctrine
• knock-and-announce

• “an Englishman s home is his castle” in Longman Dictionary of Contemporary English, Longman.
• “an Englishmans home is his castle”, in Collins English Dictionary.