Theory:

traveller who was travelling on a horse comes across a house located in the middle of the wood. He knocks at the door, enquiring if there is anyone inside. It was late in the evening, and the door was visible because of the moonlight.

The horse was too tired and hungry. While the traveller was knocking at the house silently, the horse grazed the grasses nearby. It was a forest full of ferns. There was a bird that flew out of the roof above the traveller's head.

 image60.jpg

The traveller knocked on the door once again asking if there was anyone inside. Still, no one answers, and the door remains closed.
 
No one came to open the door, even though the traveller knocking the door again and again. The traveller couldn't see anyone even on the window sill above. He expected someone to lean over the window sill, which was full of leaves and look into his eyes that was grey. Here the poet says the traveller’s eyes being “grey" which portrays his emotions. The traveller was expecting someone to help him. The traveller was confused and still standing there.
image58.jpg
 
Since no one is opening the door, the poet says the only possible listeners are phantoms or ghosts that live in that abandoned house. Hence, the house remains abandoned the traveller says that the phantoms or ghosts are the only inhabitants of the house. The ghosts stood listening to the voice of the traveller from the world of human beings. They go within the empty hall and take to the stairs as any normal Human beings would. These ghostly forms do not make contact with their world. They are helpless to reply to the “Traveller’s call,” although they can hear it. The air around them is moved and swayed by the sounds made by the alone travellers call.