There are no light sources in the upper floor other than what you're carrying, and you'll need a light source to move at normal speed. Only the squares directly in front of the windows are lit by moonlight. However, you can feel your way along a wall in the darkness if you don't want the guard to notice you're going to Aly's room. The Mobility TN to do so is only 5 if you move reeeeally slowly, and it's 15 if you try to move at normal speed. You also have to make a Stealth check, but since the guard isn't very near, the TN won't be very high.