Take a corridor from the Defense Against the Dark Arts classroom to the nearest flight of stairs leading down; McGonagall's door is on the left on the first floor. Her office has a fireplace with a mantelpiece, and the entrance is evidently within easy walking distance of Myrtle's bathroom. At least one of her windows overlooks the Quidditch pitch and some of the corridors along the route from her office to the entrance to Gryffindor Tower have windows overlooking Hagrid's cabin.
