The lead designer and creative director of Rise of the Triad was Tom Hall, and the game was his first project after Doom. He had been fired from id Software by John Carmack in early August 1993 due to differences in design. He immediately started working on Rise of the Triad, and had written the original design document by November the same year.
The game was initially meant to be an official sequel to Wolfenstein 3D, appropriately called "Rise of the Triad: Wolfenstein 3D Part II". Apogee had licensed the Wolfenstein engine for the game, but the development team added many features to the game, among others, distance shading, jumping, gibbing, parralax skies and so on. The major drawback of using the Wolfenstein engine was the lack of a z axis, a technical limitation that resulted in maps with no height differences, and a strict 90º angle architecture.
Relationship to Doom
Tom Hall brought a lot of design elements and ideas with him from id Software. Having been the lead designer for what was intended to be the retro-prequel; Wolfenstein 3D; he originally planned to migrate many of the features he designed into Wolfenstein, such as pushwalls to obtain secrets and bonus point items.
The continuous world design feature Hall argued for inclusion in Doom was also originally intended to be included in Rise of the Triad. Hall also included the characters from his Doom Bible as playable characters in the game.
Rise of the Triad contains code taken from Wolfenstein 3D, as well as early versions of the Zone memory and WAD file systems from the Doom source code. It also makes use of an early version of Doom's patch_t image format which is close to that used in the press release beta version, indicating that Hall's team was given access to an early transitional source base, perhaps related closely to other contemporary games such as Shadowcaster.
Oddly, the same zone and WAD file code, further modified, also found its way into the source code for Duke Nukem 3D, quite possibly in violation of its original licensing terms. id Software is not known to have ever taken any legal action in regard to this possible misuse, however.
Change in design
When Doom was nearing completion John Carmack revoked the license and changed the licensing terms to the extent Apogee could not market the game as a Wolfenstein sequel, purportedly to lessen the impact Rise of the Triad would have on Doom sales. The development team thus had to scrap most of the work done to that point and redesign the game as a stand-alone shooter.