"There is a 50% chance that [the NPC] is here; otherwise he is on the next level up."
What? What possible use is that randomness, other than to give the DM the illusion that the encounter is somehow more organic than it is? The players -- the people who actually matter -- are never going to know. All they will know is that the NPC in question appears somewhere inside the clocktower. They will neither know nor care that he could have appeared somewhere completely different, in a fashion that will have no impact whatsoever on the game.
Lose the randomness. No-one cares. What the adventure *should* have done, is provide several locations where the NPC could appear, and then leave it to the DM to decide. He's the guy running the encounter, he'll know the most appropriate, or most dramatic, or simply the coolest moment to introduce the NPC... and y'know, despite the fact that the adventure wants to let the dice decide, I'll bet 9 out of every 10 DM's would just choose for themselves anyway.