1. Literally, outside one's door. I'm at your door right now—where are you?

2. By extension, one's responsibility. That issue is at the police commissioner's door now. I always hire reputable contractors for my company because I don't want complaints of shoddy construction at my door.

3. Very close to one's location, especially of an unwanted thing that is encroaching. We used to think of it as a skirmish on the frontier, but the war is now at our door. This whole area used to be rural, but with rampant development, the suburbs are at our door.