Advent Door is copyright 2019-20 by Andrew Plotkin (firstname.lastname@example.org). It may be distributed for free, but not sold or included in any for-profit collection without written permission from the author.
This game was written for a collaborative 2019 Advent Calendar project. The theme was "The City of Doors" from the Planescape roleplaying setting. That project was never completed, so I released this game on February 29th, 2020. (An appropriate date for a misplaced day of the month!)
The only other released Advent Calendar game is Gateway of the Ferrets by Feneric.
Thanks to Feneric and the PR-IF meetup group for playtesting.