[CyberDildonics]

Pretty sure water was just a teleporter? You may be thinking about the mirror effect?

That's what they just said? They didn't mention the mirror effect? The mirror effect was done with duplicate geometry? It occupied the overlapping space of what was behind it? The build engine also didn't have vertical levels.

[paulryanrogers]

Shadow Warrior (Build) did have some room over room stuff. But IIRC it still leveraged offset sectors like Duke's water. FWIW some modern Doom ports appear to have this as well, at least with levels that support it.