The toughest question was the artwork. Since Wolf was selling well, I had planned on paying contractors to upscale all the Doom graphics to twice the original resolution. When I pulled all the graphics out and tallied it all up, it looked a lot more marginal than I had expected. There were over two thousand individual pieces of art, and it was over ten megatexels in exactly bounded area, let alone atlas fit or power of two inset. The PVRTC compressed formats would work great for the floors and ceilings, which are all nice 64x64 blocks, but it has issues for both the walls and floors.
PVRTC textures must be power of two and, notably, square. If you want a 256 x 512 texture that needs to repeat in both axis, you need to resample it to 512 x 512 to use PVRTC, which means you lose half your compression benefit and get distorted filter kernels and mip level selections. Even worse, Doom had the concept of composited walls, where a surface was generated by adding, say, a switch texture on top of a wall texture. You can't do that sort of operation with PVRTC images. The clean sheet of paper solution to both problems is to design around textures that the hardware likes and use more geometry where you need to tile or combine them, but Doom wasn't well suited to that approach.
Character sprites don't get repeated, so a lot of them can be packed into a nice square 1024 x 1024 texture to minimize waste, but the PVRTC texture formats aren't well suited to sprite graphics. The DXT1 compression format has an exact bit mask for the alpha channel, which is what you want for an application like this. PVRTC treats alpha like any other color channel, so you get coupling between the alpha and color channels that results in partially transparent pixels ringing outside the desired character boundary. It works fine for things like clouds or fireballs, but not so good for character sprites. It looks like it should be possible to get an exact binary mask with the 2 bit PVRTC mode, which could be combined with a 4 bit PVRTC color texture to get a 6 bpp perfectly outlined sprite, but the multitexture performance on the iPhone, even with PVRTC textures, is not fast enough to prevent missing 30 fps when you have a hoard of monsters in front of you.