Why would you want to break down a castle door? Why not just knock politely and ask to be let in? People are usually pretty nice if you ask politely, especially if it's raining or something and you just want a place to keep dry, even if it's just under the eaves or in the barn. But you have to remember to close the barn door when you leave and it's nice to maybe help with the milking or hog slopping or stall mucking-out. Besides, doors can be expensive and if you break one down you'll have to pay for it, just as if you broke a window throwing rocks or playing baseball. I know that if I owned a castle and someone broke down my door I wouldn't be happy about it. I'd probably call the cops, because breaking down some else's door isn't very nice.

I don't know about breaking down your own door. That might not be impolite, but it could certainly be stupid. Unless you forgot your keys, of course. Then you could break down your own door because you had to get into the house and maybe there wasn't anyone inside to let you in. So that might be okay, but it was still pretty dumb to lock your keys in your house.