I built a simple clone of the portal rendering technique made famous in Portal in Unity (the red color is to make the portal itself more visible and wouldn't be used in a proper game). It has some limitations, mostly graphical due to the oblique projection matrix of the portal camera. Shadows in forward rendering used to work prior to Unity 5, however now they don't work at all. Of course you can still use lightmapped shadows.
A fun effect, not as complex as you'd might think once you know what you're looking at.