>>12164537
Correct, Doom is considered 2.5D for the reasons explained above. It can't show arbitrary polygons, it's just using a sprite scaling technique to make vertical walls appear like angled textured surfaces. It's less of a 3D game than something that only has gouraud shaded polygons like Star Fox or Virtua Racing.
But this Sonic game I wasn't familiar with until now does seem to be real 3D. It's not comparable to a 2.5D beat em up so I don't know why that other anon said that.