Far Bombsite:
Having got up to the balcony level, CT's may not enter the doorway into the corridor (that ultimately leads to T spawn).
Spawn Bombsite:
CT's, having gone down the slope from spawn (middle route) may not pass the second set of doors (under the sniper windows).
CT's may enter the corridor route to the spawn bombsite, but may not go so far down the corridor that they can see into the courtyard that the sniper windows overlook.
All rules are off once the bomb has been planted.
(http://www.btinternet.com/~p.d.herbert/mhpcs/2dmaps/CBBLE.JPG)