Well, one more thing I just checked again: there was no Siege Doors, only a Porticulus. I am speaking about this small room leading from the Empire lair to the winch. I actually managed by repeating my route again with Archon to glitch inside through the wall.

"but can place them in FoW."

Yeah, what you say is right, but ^ if you do it in FoW it is possible even if there is a wall here actually.

Wait, what? Sorry for bringing this from the dead, but eh, I literally spent like half an hour trying to figure what's going on at this particular place of the level. What's the deal? Can you elaborate why is this made so?

Same issue here on a standard scenario/sandbox map, my imps would take and instantly drop my fallen minions, leaving them to die.There was a path to all lairs, even tho not straight.