Text Mode Doom

Text mode renderers are a feature which open source hackers find bizarrely novel to incorporate for applications which would traditionally be best suited to a more detailed interactive display. Doom is no exception, and there have been a number of attempts to develop a text mode renderer for it.

FastDoom
The source port FastDoom supports text mode rendering since version 0.8 as part of its goal to render as fast as possible on 386 and 486 class hardware. It is using two separate binaries:
 * for CGA, EGA and VGA video cards in 80x25 resolution in 16 colors in a virtual 80x50 resolution
 * for VGA only video cards in native 80x50 resolution

It also internally converts the 256 colors to the 16 colors suitable for a text mode screen.

SMMU
The Smack My Marine Up (SMMU) source port incorporated a DOS text-based renderer.

libcaca
Libcaca is a text-rendering library that can be plugged into a variety of other programs and renderers, including libSDL. Doom was used as a demonstration of the library's novelty. The second image below is from Doom Legacy while the first image may also be from Doom Legacy but there is not enough information in the screenshot to determine if it is.

libcaca support was removed in SDL 2.0, so you'll need to find a source port release based on the older SDL 1.2 codebase. For example older versions of Chocolate Doom (2.x series) are based on SDL 1.2 and will work. You may have to compile yourself.

To run once installed, set the  environment variable, eg. export SDL_VIDEODRIVER=caca chocolate-doom