Used other than figuratively or idiomatically: see at, the, door.
Ringing the doorbell or knocking on the door.
The delivery man is at the door.
At the first threshold of the event itself rather than in advance.
You can buy tickets at the door, but they will be pricier.
Doubt comes in at the window when inquiry is denied at the door. Benjamin Jowett
Luxury is the wolf at the door and its fangs are the vanities and conceits germinated by success. When an artist learns this, he knows where the danger is. Tennessee Williams
I have always knocked at the door of that wonderful and terrible enigma which is life. Eugenio Montale
When in someone else's home leave your defects at the door. Rwandan Proverb
It is useless to knock at the door of a deaf man. Greek Proverb
He knocks boldly at the door who brings good news. Italian Proverb