Hi,
Isn't there a switch in one of the round huts in the first level to open that door ? I think it's in one of the round huts, one that sits on a dead end.
Edit: Ok, so i revisited Channelwood to make absolutely sure and here it is,
With your back turned to to the door blocking the spiral staircase to ground level,
-go forward to the squared hut,
-turn left and go forward to another squared hut (3 or 4 baskets hanging from the ceiling in this hut)
-keep going forward to the round hut
-turn left and go forward to another squared hut (there's alot of pottery in this hut)
-turn left and go forward to another round hut,
-turn left and go forward to another round hut, the switch is on the right side of the hut as you walk in
When you use the switch you'll see the door unlocking and opening in the distance.