I made the lamp shade from a bottle cap. This is as much or as little that will be visible through the door. However, both ends of the corridor can be seen through the side windows. It's almost impossible to take a good picture, or any picture, but I promise, it looks just as mysterious as I want it.
There are two pictures on the wall, and of course there is a cat at the bottom of the stairs. I have tried rugs, but space is so tight that a rug killed it. But I did put in a bit of skirting, even if it's just barely visible, and there is a proper newel post.
From the opposite side, there is this wonderful under-stairs space, far too small to make much of. But enough to put in a chest of drawers. I first tried a simple white one, from my supply of Poundland furniture. But it felt too plain. I painted it green, same colour as the walls, and then distressed it with a coat of lighter green which I wiped off with a cloth. I used bead fittings for handles.
Now it looks sufficiently worn-out (maybe slightly too worn out, but it will only be visible from the side).
The light is temporary, "borrowed" from the kitchen, just to test the effect.
Now you may ask, why do I put so much energy into something that won't be visible except if you know where to look? It's a bit like writers putting in family jokes into their books. Or painters putting the same tiny detail in every painting. Nobody will notice, unless... But I know that there will be at least some keen eyes who will discover the secret corridor, and for them, everything should be done properly.