Luke wanted to have another look at the Turkish Lounge right at the top of the tower. He'd liked that best; chambers straight out of Sindbad's Adventures.
"You should have thought of that earlier," his father said. "They're closing in twenty minutes. We haven't been to the turret yet."
|Turret, Oberhofen Castle by the Lake of Thun|
So Luke had fallen behind when his family was strolling through the Summer-Room in direction of the turret.