Some of the doors were locked, some of the time.
Think of the french doors/windows in the sitting room...
True - though mostly when the plot dictated it. At other times people came right through the window.
Barn did usually lock his door; I know this because of all the times people knock and knock and knock....
Yes, when it comes to the Old House, its door is often another story entirely. Though even at the Old House there are many instances where people just walk right in. And let's not forget how many times someone gets in through an unlocked window. It's not very smart to lock the door but leave the windows unlocked - but that's our Barn...
Though when I posted I was thinking mostly of Collinwood's main door. There may be instances when someone actually had to unlock it to let someone in - but 99.99% of the time it was left completely unlocked. And as dom pointed out, it wasn't even just the family who'd walk right in. I'm reminded of Adam sneaking right in with the intent of killing Vicki after the experiment failed. Though it would have been hysterically funny if he'd knocked, someone had answered, and he'd said to them "May I please come in? I'm here to kill Vicki. Have a pleasant evening."