The descent gully can be difficult to find if there are no tracks in the snow and snow lingers on the summit ridge for much of the year.
You are looking for a gully north of where you top out on the summit ridge. Descend and past the first notch and gully you come two. Do not get sucked down the east slopes of Castle Mountain by cairns that represent the scramble route up from Rockbound Lake and exits a completely different side of the mountain. After descending to and passing the first notch, angle straight across or up searching for the next gully which will not come into view until you reach it.
Castle Mountain, after completing the Bass Buttress Direct Route, II, 5.7, July, 2007, Banff National Park