Franz Polzer, head of the criminal investigations in Lower Austria, said: "There is not only one, but a number of rooms: one room to sleep in, one to cook, and there are also sanitation facilities."
 
He said that the small alleyway, with uneven floors, leading to the rooms had been added on.
 
The woman, 42, and identified as Elizabeth F, has told police that she had been abused since the age of 11, seven years before being imprisoned.
 
'Massive crimes'
 
She accused her father of "massive crimes."
 
The 73-year-old man allegedly lured his daughter into the basement of their home in the town of Amstetten, anaesthetised and handcuffed her and then kept her captive.
 
A police statement said: "She had been abused continuously during the 24-year-long imprisonment.
 
Elizabeth F said that her father provided food and clothing for her and three of her children, who were kept in the basement with her.
 
Rosemarie, his wife, was apparently not aware of the events.
 
Babies 'left on doorstep'
 
The woman went missing on August 29, 1984, Lower Austrian police said. Interpol, the international police service, had opened an investigation into her disappearance.
 
A letter in her handwriting had been sent to her parents requesting that they stop searching for her. Local authorities believed that she had been taken by a religious sect.
 
Austrian investigators were told how to find
the basement by Joseph F [Reuters]
In 1996 she gave birth to twins, but one of them died several days later due to lack of appropriate care. The body was subsequently burned.
 
There are believed to six surviving children - three boys and three girls - all seemingly born in the bleak captivity.
 
Joseph F legally adopted two boys and one girl. He told his wife and local authorities in different years that Elizabeth had left the babies on his doorstep.
 
A letter from Elizabeth accompanied each delivery, saying that she could not support the child as she had others to care for.
 
The three children went to a local school and were apparently unaware that their mother and three other siblings were living below them.
 
The children are aged between 5 and 19-years-old.
 
Neighbours and social services in Amstetten do not appear to have had any idea of what was happening.
 
'Greatly disturbed'
 
Elizabeth F was "greatly disturbed" during questioning, police said.
 
She would only talk to officers after being assured that she would no longer have contact with her father and that her children would be taken care of.
 
Her case came to light after one child, identified as 19-year-old Kerstin F, was taken to a local hospital after falling severely ill.
 
A Lower Austrian police spokesman said: "A 19-year-old girl was dropped off at the Amstetten hospital last weekend.
 
"The girl is seriously ill and is fighting for her life. A search for the mother, who had disappeared, was launched to get more details about the girl."
 
Elizabeth F was found by police on Saturday night following a tip.
 
An investigation has been launched and Joseph F has been arrested, although he has not confessed.
 
DNA samples of all those involved have been taken for evaluation.
 
It is the third shocking case of children being held captive in Austria in recent years.
 
Natascha Kampusch was kept in a neighbour's basement in Vienna for eight years before escaping in 2006.
 
Another case saw a mentally ill mother lock up her three daughters for seven years in the city of Linz.