It's the bottom of the ship; that's where the boarding party beamed in at, it is pretty specific in dialog
Where the boarding party beamed in and where the fight took place must
be two completely different locations - the Remans would not stand still. We can see that both sides in the conflict proceed on foot, so a meeting at the halfway point is rather likely.
Furthermore, the fight continues after the two sides meet; it takes a long time for the action to move to the bottomless shaft. All that time, the Viceroy would be heading up; Riker would be desperately trying to catch up with him, but the Reman would have no motivation to loiter and deal with the Commander when his logical course of action would be to leave Riker far behind (that is, below).