I think you might make the lower level darker because as soon as the shadows of the stairs end it becomes as bright as the upper level. This blows the descending look you have going. If this is in a larger map and you don't want to show the entire lower level in a darker shadow you would have to have a light source, such as a torch or something. You might also be able to pull it off by just darkening the shadow for a ways down the hall and gradually returning back to the same shade as the upper level but it might just look like a ramp.

That's what I would try anyway. If I have time tomorrow I will try to show you what I'm talking about in case I'm not making sense. I'm a little brain shot right now.