Hallow Hall is the home of Prince Cardan and Balekin. The front door of the mansion is described to have an enchanted door with eyes and the ability to talk. It recognized Cardan as its prince and Jude as its prince's lady.
The first time that this tower is mentioned is after Jude has been recruited to be a part of the court of shadows. She must go there on a mission to collect any evidence that Prince Balekin is doing bad things. She finds a letter that talks about a poisoning. While she is up in a tower, Prince Balekin and Cardan enter the room and Jude is forced to hide under a chair. There she sees Cardan beat by his brother for under performing in his swordsmanship.
In The Cruel Prince, Cardan asks for authority over Hollow Hall as his conditions for agreeing to oblige to Jude for a year and one day.