'Fortified' in the Bible

‘If a man sells a house in a walled city, then his right of redemption remains valid for a full year after its sale; his right of redemption lasts a full year.

But if it is not redeemed for him within a full year, then the house that is in the walled city passes permanently and irrevocably to the purchaser throughout his generations. It does not revert back in the Year of Jubilee.