How does Roald Dahl create tension in the story "The Landlady"?

In this story, Roald Dahl's "The Landlady," tension is
created by the use of suspense. We know from the very beginning that something is not
quite right. Billy has every intention of going one place and ends up in a totally
different place after experiencing some kind of hypnotic moment. We know that things
just are not quite right at the Landlady's house, as well. Umbrellas in the stand but no
guests in the house. A bed turned down and a room waiting for him when he had no plans
to even be there. Cryptic remarks by the Landlady throughout Billy's time there.
Recognizable names in the guest book--with no evidence of their having ever left. From
beginning to end, something is just not right. It is this which creates the tension
which builds to the rather shocking--though not all that
surprising--climax.