Build a large victorian house which contains in one room a large mirror over a dormant fireplace.

But the mirror isn’t actually a mirror— it is a hole in the wall which opens into an identical room on the other side, hidden within the architecture of the house.

When someone looks in the mirror, they will see everything the same except they won’t be there. They can then climb up and ‘step through’ the mirror into the alternate room, and potentially explore the alternate reality grounds.

(Alternately, it may be cooler to put up glass over the window, so it really DOES look like a mirror, making the ‘no reflection’ effect more freaky. And place the mirror physically out of reach.)

(There would have to be objects placed on the mantle right in front of the mirror to sell the effect.)