Short answers, pulled from the story.
The first episode of Doom was uploaded to the internet on the 10th of December 1993. The development team at id Software worked for thirty straight hours to complete the upload at midnight.
Lead programmer John Carmack developed the new engine that rendered 3D graphics while keeping enemies and objects as 2D images. This technique known as 2.5D graphics allowed walls and floors to exist at any angle or height.
Lead designer Tom Hall was fired in July 1993 after internal conflict regarding the game's story and design. John Carmack dismissed the importance of story, leading the team to focus on technological innovation and fast gameplay instead of Hall's science fiction plot.
Adrian Carmack and Kevin Cloud sculpted models of enemies and took stop motion photos from five to eight different angles to create realistic in-game rotation. Developers also photographed themselves and children's toys to create weapon textures and character models.
The swastika-shaped structure in level E1M4: Command Control was removed in later versions out of respect for a military veteran's request. This change was made by Romero after the initial release of the game.
The source code for Doom was released under a non-commercial license in 1997 and freely released under the GNU General Public License in 1999. This release allowed for unofficial ports to esoteric devices such as smart thermostats and pianos.