He needs no rope to help him reach the window, he has a spell to float himself up the wall. He needs no cover of darkness, he has a spell to render his form invisible to the guards. It is night, though, because that is when she asked him to come.
The Princess is waiting at the window. She cannot see him, but she hears the rustle of his clothes. "My father is asleep."
"Heavy snores the head that wears the crown?"
"Something like that. And…" She pushes a strap of her dressing-gown from one shoulder, and grins. "Stay invisible."