Stop Them!
As for leaving this place and joining them up there, it was pointless. First, they didn't have much force to make a difference up there. Not to mention once they left here, it'd be hard to return back down here.
If the enemies noticed their intentions to come down here, then they'd rally everything they had, and fortify the landing points at this place. So it'd be like falling directly into the enemies' trap.
The only path ahead of William was by cutting any way for the enemy to reach the surface. Then the enemy would have to activate the switching and portal ground formation up there at the ceiling, move the masters down here, and let them face the enemies coming out from the two gates.
At the same time, the endless enemies that went up there would return back through the holes. It wasn't the best approach for the enemy, and that was what made William take such trouble in handling the bridges before doing anything else.