An interesting situation, Sorry if it's been asked I cannot find it and will delete this post if someone can supply a link to an official answer.
My opponent put a Troll Trap marker on the path from the Rampart to the Wall. The wall has a maximum occupancy of three.
Can I move 4 units which would include... say, two Trolls and two Orcs knowing that only three of the units will make it to the wall (because one Troll will get caught in the trap) to occupy the three available spaces?
On page 12, it says that there is a limit of units "PLACED ON IT", not that can travel on the path. The trap eliminates a troll and then you fulfill the requirement for the number of creatures placed in the spots.
Looking at it from a fairness standpoint, I would say that should be the way it works or else the trap would stop 2 trolls from advancing (your dead troll and the one that would make it with the rule listed above, but can't because of the placement limitation)