I was wondering how you did the portals so seamlessly and perfectly even with sound and stuff working... Building your own engine explains it.
It was the most important reason why I decided on my own engine. Fast portals rendering, sounds, navigation, AI, object interaction, physics (although physics is most problematic as I decided to do it quite experimentally with soft collisions).
I've seen people doing portals with both Unity and Unreal but I heard that it slows down significantly. Not mentioning about other things. I think that it might be possible to do it with existing engines although you would have to heavily modify bits of it. This of course means that every bigger update could induce chaos.