The way I understand the area B3, the tower of perpetuity, is if the PCs try to walk the stairs, they basically won't get anywhere, coming back to the start. To reach either the hallways or the landing to B4, they have to jump into the shaft and try to land at those areas(DC15 reflex after 2 rounds). In fact, they allways suceed in jumping on the hallways, but they can take damage, can’t they?
You can walk the stairs to reach the hallways or the landing to B4.
If you walk around the stairs in either direction, you will reach your original starting point, but you do reach the branching hallways on the way.