Thanks :)
That may sound weird but I was amazed three times when I played my game:
1. when I tried impossible spaces within my bedroom, the smallest playarea there was, 2x1,5m. It felt good.
2. when I bought tpcast and went wireless. 2x2m but it was enough to feel free with no cable.
3. recently, when I played outside with 8x6m. It felt great. The place was huge and seeing a wide corridor going with a slight turn for 5 meters made me run through it.
As for tutorials, how to achieve that. I have no idea if there are any actual tutorials for impossible spaces but I've seen vidoes by people who did portals in their own engines, Unity and Unreal. The most low-level thing you may want to learn is Stencil Buffer. This is a kind of a buffer that allows you to decide where you draw stuff on the screen and where not. I use it to draw portals (increasing stencil value by one with each nested portal). You may also need to use clip-planes to clip objects that are crossing through a portal (to draw a part of an object on one side and then another part on the other side, through a stencil). There is also another way to do portals, it is with a use of rendering to a separate texture. But it is a much slower approach. I'd recommend to stick to Stencil Buffers. Oh, there is also another way, using clip-planes alone but on most of the hardware there is a limit of active clip-planes and it is quite low (6).
The things start to get tricky with physics, sound, interactions and AI.