As the rope descends, an odd thing happens. You see the bottom of the rope dip down 15 feet, then 30 feet, and looking up, you see the same rope dropping down toward you. This appears to be an endless loop of holes in three floors, each floor being 15 feet.
In other words, after about 45 feet of dropping the rope, the top of the rope is dangling in front of your face.
Taking a minute to look more closely, you see that the only difference between the rooms is the placement of the southern door on one of the rooms and the color of the ring around the hole. Yours is black,the one below you is red, and the one below that is white. Looking up, it appears that the one above you is white, and the one above that is red.
Below is a diagram. They are supposed to be all the same size, so ignore my error. You are at 20a; you can reference the others by 20b and 20c or by color of the ring.
Grizz tries the door to the west. It is unlocked.