We don't have a huge bedroom but wanted to add a walk-in closet. We placed the IKEA wardrobes so the window could be opened. Because of the window, we could not add a door (which is not too bad).
Here you can see how we placed the wardrobes (called PAX) in a kind of U-shape.
Add chipboard to the wardrobe and put wallpaper on it. Paint it in whatever Color you like - your landlord won't mind because ist can be easily removed.
You can see a litte better how it was done in our other room where we disguised a shelf with the same method.
From the outside is will look like a normal wall, you can even hang pictures or mount something else.
Even from the bed you don't see that it is a wardrobe (but some neighbours have a clear view of our underwear...)
We added lights triggered by a motion sensor on the inside. As long as you move, there is light. One minute after leaving the light is switched off auomaically.