(CNN)  -- Austrian investigators Monday released more details about the elaborate underground cellar where Josef Fritzl kept his daughter imprisoned for 24 years, along with three of their children.



Josef Fritzl admitted to authorities he raped his daughter and fathered her children.



Investigators believe Fritzl planned to build the cellar as early as 1978, shortly after, according to his daughter, he began raping her at age 11 or 12, said police spokesman Franz Polzer.



The 73-year-old Austrian began building the dungeon as part of an addition to his home that year, and simply added the hidden space -- which was not recorded in any building plans -- Polzer said. It took Fritzl until 1983 to finish the addition, Polzer said.



Investigators recently discovered another door to the dungeon prison, which was blocked by a 500-kilogram (1,100-pound) steel and concrete door that Fritzl probably stopped using when he later constructed an electronic door for a second entrance, Polzer said.



Fritzl, who police believe was the only one with access to the cellar, had to travel through an elaborate maze to get to the prison.



"You would have to open up a total of eight doors, and ... (for the) last door which would go into this space (where the family was imprisoned), you would also have to use electronic opening apparatus," Polzer said.



"We will have to find out perhaps later from now if perhaps there are other spaces we haven't discovered yet, and perhaps maybe there is something else interesting."



Fritzl was recently arrested and confessed to holding his daughter, Elisabeth, captive in the dungeon under the Fritzl home for decades, repeatedly raping her and fathering seven children -- six of whom survived. Three of the children were adopted by Josef Fritzl and his wife after he concocted the ruse that Elisabeth had left the babies on their doorstep.



The story of the family's imprisonment began to unravel more than two weeks ago, when one of the children still in the dungeon, 19-year-old Kerstin Fritzl, fell seriously ill with convulsions.



The father agreed to take her to a hospital, the first time she was allowed out of the prison where she had spent her entire life with her mother and two brothers.



Dr. Albert Reiter, who is treating Kerstin, said Monday that while her condition is still "grave," it "has improved somewhat."



"She has become more stable, but despite that we have to continue to keep her under sedation and give her respiratory help," Reiter said, noting it is not clear how long she will be kept under sedation.



Elisabeth and her two sons were reunited with her mother, Rosemarie, who police say knew nothing about the basement prison. They were also reunited with the three children that Josef had taken from Elisabeth. The reunited family is living in secluded quarters at a psychiatric clinic, where they are finding a daily routine and adjusting to sunlight -- something the two boys had never seen -- according to the clinic's chief doctor.



"The mother and the smallest child have, in just the last couple of days, increased their sensitivity to light," Dr. Berthold Kepplinger said. "So we have been able to equip them with protective sunglasses."



Five-year-old Felix is "getting more and more lively," Kepplinger said.



"He's fascinated by everything that he sees around him -- the fresh air, the light, and the food -- all of these things are helping them," he said. "Slowly the color of their skin is changing back to a more normal (shade)."



He also said the family members are still getting to know each other and live together as a family.



Kepplinger praised Elisabeth for having provided a daily living routine for her children during their captivity. He said the family is getting into a new routine in which the mother and the grandmother make breakfast for the family, and the children make their beds.



However, he said there is a noticeable difference between the pace of life of the children held in captivity and that of
Who gave this interview? Who's condition is grave but stable? What is the daughter's condition? Who was hospitalized? Who imprisoned and raped his daughter?
Franz Polzer. Kerstin Fritzl, "grave," Kerstin Fritzl, Josef Fritzl
question: Who gave this interview?, answer: Franz Polzer. | question: Who's condition is grave but stable?, answer: Kerstin Fritzl, | question: What is the daughter's condition?, answer: "grave," | question: Who was hospitalized?, answer: Kerstin Fritzl, | question: Who imprisoned and raped his daughter?, answer: Josef Fritzl