Up you go

You step inside the stairwell and let the door gently close behind you. When it clicks shut, you turn around for a moment. The only thing breaking up the plain wall is a plaque inscribed with a small "1".

Turning back to the stairs, you take some time now that you're alone to check the stairs down for a moment. You take one step down and lean over the railing, trying to see how far down it goes. To your surprise, you see the stairs go at least several levels down, but you can't see the bottom. Before your curiosity gets the better of you, you turn back to the stairs up.

You take a deep breath, counting the steps ahead of you. Ten steps, up to the next landing. You turn to the next flight of stairs, the next door coming into sight as you ascend the first step. At the top of this flight, you're face to face with the door. The plaque next to it reads "2". You turn to the next flight. Two more flights, ten steps each, before the door marked "3".

This will be a long trip up. No wonder your guide had to go back.