If you could descend from the gallery by way of the window, by means of the terrace, then you could get into the chateau by the same means.
"The murderer was still in the chateau, for here were marks as of returning footsteps.

He had entered by the open window at the extremity of the 'off-turning' gallery; he had passed Frederic Larsan's door and mine, had turned to the right, and had entered Mademoiselle Stangerson's room.

I am before the door of her ante-room--it is open.

I push it, without making the least noise.

Under the door of the room itself I see a streak of light.