Created a valid room upstairs, don't know how an NPC can go back down after successfully going up

So I created a few houses, some are upstairs that they're not on the ground. So I put wood platforms as stairs to let them go up. Soon a merchant moved. I can't seem to let it go upstairs to their room when night falls, but that's not the real problem, because after exploring a jungle, I went back to my house and found the NPC upstairs, but I don't know how to let it go down to the ground. Appearently I explored one more time but the NPC was still upstairs. I was also wondering how I could make the NPC go up and down the floors while I'm at the house with the NPCs' rooms. It seems that the merchant couldn't use the wood platform to go up to her room (but when I explored a random area and back, it was upstairs). But I was thinking of a way that my NPC could go back down to the ground WITHOUT having to explore somewhere else. Is there a way to let the NPC go back down? Are there any items I could craft? Are there like ladders in Terraria? Sorry if I don't have a screenshot but if you understand me please answer.

The NPC is not the Guide. It's a painter that sells painting materials.

Npc's are capable of a lot more things when you are not looking. When a suitable distance away from your home, NPC's will teleport to their designated home, even if it is otherwise impossible for them to reach the house.

As far as wandering npc's go, I believe that npc's don't mind dropping down a few blocks, as long as they don't need to go through a platform. Making a balcony would probably work. They would jump of the balcony on the ground outside, then wander around outside.

_{(Note: Answer is for the PC version, as one of the tags you used is for the PC version. I have no experience with the other versions.)}

The issue isn't with the stairs, it's the platform flooring that the stairs go through. We can fix this by not having the stairs lead to a platform floor. Instead, put the stairs on the outside of the house, with a door so the NPC can leave the house horizontally and walk down the stairs to reach the ground. Alternatively, wire up a teleporter from the room to the ground.