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...)