Elisabeth l, now 42, was kept as a prisoner by her father Josef for 24 years. He raped her repeatedly, fathering her seven children, one of whom died.
In statements to a judge compiling evidence against her father, Elisabeth reportedly said that if she refused Fritzl sex, he would kick and abuse her children.
'He was very brutal against me.
'When I did not agree to have sex, then the kids would suffer. We knew he would kick us or be bad to us.'
Fritzl also threatened to leave her and her children to rot behind the locked door and bullied the children when they dared to answer back to him.
She said: 'He would often say we had no chance down under, in the cellar where it all happened.
'He said he could close the door whenever he wanted and then we would see how soon we survived.'
She tried to make as normal as possible for her children in the cellar, singing songs to them and telling them stories.
But whenever Fritzl entered the cellar, she said, the atmosphere would change, Sun newspaper of UK reported her as saying.
'When he went away we led our own lives,' she said in the statements. 'When he was down here it was all silence. When he came down to the cellar we just tried to survive.
'He was all-powerful.'
Austria's most senior female judge, Andrea Humer, asked Elisabeth the series of delicate questions as part of the case against Fritzl.
She asked: 'You tried to make life as easy as possible in order to stop him being dangerous?'
Elisabeth replied: 'Yes.'
She told the judge how she feared to leave her children alone with Fritzl.
She continued: 'It was his kind of communication to use rough words. He would be insulting against me and the children.
'When he said such words against the kids they ducked and tried to get out of his way. He would say, 'Shut up and get away from me.' And if that wasn't enough he would be even more abusive.'
Fritzl became more controlling as the children grew older and developed their own personalities, he said.
'When he was at the table and we were eating and someone was holding their knife wrongly, or did not want to eat, there would be verbal abuse. Or even if you walked wrongly or in a way he did not like.
'He wouldn't let the kids develop their own personalities. He didn't like them to talk back at all.
'At the beginning, when they were small, it wasn't such a problem. But as they got bigger and started developing a personality, it was more of a problem.
'He did not like it and he tried to stop it. He would not allow the kids to have their own will.'
Fritzl's trial is expected to take place later this year.
Meantime in Poland, the other abused girl, identified as Alicija B said that she was 15 when her father first raped her.
Over the next six years, she fell pregnant twice and was kept tied up in a locked room with no door handles.
She was taken to different hospitals to give birth while her father stood guard over the bed.
He then made her abandon her children in the maternity ward. 'It was all horrific,' she said and added, referring to his recent arrest, 'When I saw him in handcuffs, I just sighed with relief.'
It has now emerged that the victim's mother knew what had happened but was repeatedly beaten by her husband to keep her silent.
The man has been identified only as Krzysztof B in keeping with Polish privacy laws. Police believe he was trying to flee the country when he was stopped.
Polish police will conduct tests on the boys to determine if the man is the father and are investigating whether to bring formal charges of forcible incest, regional spokesman Jacek Dobrzynski said.
The man will be held for three months during the investigation, after which the court can extend his detention.
Apparently the girl was let out of the room on occasion, including for the births of her two sons in hospitals.
The first boy was born in February 2005 in the southwestern city of Wroclaw, and the second in January 2007 in the northeastern area of Siemiatycze, where the family had moved.
Neighbors say they often saw the daughter outside the family's home and that she was seen walking to a nearby store to buy food and hair dye. She also attended church with her parents and younger brother, AP reported.
It is not clear though why the mother and daughter did not come forward earlier to expose the man.
"There are very many questions in the case, and talking to the woman is very difficult because she is very emotional about what has happened to her," national police spokesman Mariusz Sokolowski said.
"The main problem was the extreme psychological pressure she was under, the intimidation," Sokolowski said. "We are also looking for people who might have known about the situation."
Adam Kozub, a spokesman for the investigating prosecutors, said they have questioned the man and his brother, who is also a suspect in the case.