Worked from memory. The stair portal isn't totally perfect, but whatever.
(Some textures come from the OTEX.wad texture pack)
@Ronflaix doom portal wad when
@tuxic Well, if you squint hard, Portal is more or less Quake with portals, right, right? :')
@Ronflaix can doom be thinking with portals
@tuxic Wait, people already went the extra mile to do it : https://www.youtube.com/watch?v=R9_YI_ex3Cs
Amazing
And as soon as I post the video, I got a solution for a seamless stair effect, go figure.