Animated wall

Animated walls are implemented using wall texture names that are built-in to the Doom rendering engine. At initialization time, the engine finds each texture name listed in the "First" column below (in the TEXTURE1 or TEXTURE2 lump), and links it with all following texture names up to and including the name in the "Last" column. At rendering time, a linked texture name in a sidedef of a wall is changed to the next linked name about three times a second, wrapping around to "First" after "Last". (Scrolling walls are another form of animation; any wall with any texture can be made scrolling by assigning the appropriate linedef type.)


 * Notes
 * 1) There is a FIRELAV2 texture in the IWADs, but due to its position it is not in the sequence defined by the table.
 * 2) There are no WFALL textures in the Doom or Doom II IWADs. The Plutonia Experiment defines some waterfall textures, and TNT: Evilution has a set of multi-purpose textures.
 * 3) PWADs can replace the TEXTURE1 and TEXTURE2 lumps, as well as individual wall patches. Thus the number of frames, and of course the description, can vary from the table above.

The following codes are used in the "Version" column above:
 * S - Shareware
 * R - Registered/Ultimate Doom
 * 2 - Doom II
 * F - Final Doom

Beyond original specifications
The PWAD, exploits a trick where more frames on an, albeit smaller framesize reference an altered Icon Of Sin wall texture with vertices, one pixel apart to play a short video clip, which was impressive for vanilla Doom during the mid 90s.

Source

 * Unofficial Doom Specs