Alpha Prime is this world you and I inhabit. It is the truest of the Earths, which is to say it is the most like and unlike any of the others. It is impossible to travel between the Tower and our Earth using the portal, which has been damaged or disabled at some point in the remote and ancient past.
It was Parciloquy’s mission for a a couple thousand years to open that portal. He almost succeeded, at the cost of human civilization. In all my time spent within the Tower, I was never able to figure out how the door to our timeline became disabled.
It is my belief, however, that our version of Earth’s lack of magic and the inoperative portal are connected. It is a mystery that will likely outlast my life or yours, so it is pointless worrying about it anyway.