Breivik trained for his attack by working out in the gym, running with a backpack filled with rocks and practicing at a shooting club, the court heard.
In the course of the 10-week trial, which wrapped up in June, the court heard chilling evidence from some of those who survived Breivik's shooting spree on Utoya Island, in which 69 people died -- most of them teenagers attending a Labour Party summer youth camp.
In his own testimony, given without emotion, Breivik recounted firing more bullets into teenagers who were injured and couldn't escape, killing those who tried to "play dead" and driving others into the sea to drown.
His fertilizer bomb attack against government buildings in Oslo also killed eight people and injured many more.
It was only luck that more people were not killed and hurt in the blast, the court heard.
Breivik blames politicians, and the Labour Party in particular, for promoting multiculturalism in Norway.
He has been held in a "particularly high security" wing of Ila Prison since his detention immediately after the killings.
The prison's governor, Knut Bjarkeid, said Wednesday that the institution was ready to hold Breivik securely whether the court ruled him sane or not. "Our job is to protect the community," Bjarkeid said.
Over the past year, Breivik has had three cells for his use, one for physical exercise and another for reading and writing, as well as a separate outdoor exercise space, he said. Breivik cannot mix with prisoners from other wings, but does have contact with prison staff.
"As of now, we think there is a need to subject Mr. Behring Breivik to a particularly high security regime," Bjarkeid said.
The high security regime "puts a heavy strain on an inmate, especially if it lasts for a longer period," he added, so Breivik's continued detention under these conditions will be kept under constant review.
Defense lawyer Geir Lippestad has previously said it is important to Breivik that people see him as sane so they don't dismiss his views.
The court had to consider conflicting opinions from medical experts in reaching its verdict.
An initial team of psychiatrists found Breivik to be paranoid and schizophrenic, following 36 hours of interviews.
However, a second pair of experts found he was not psychotic at the time of the attacks, does not suffer from a psychiatric condition and is not mentally challenged.
Their report said there is a "high risk for repeated violent actions."
Mark Stephens, a partner at law firm Finers Stephens Innocent, told CNN Friday: "The general public will think only a madman can commit these offenses, but in law madness is defined very narrowly. Basically it requires a doctor to come to court and say this person has a definable medical illness -- in this case the prosecution said he was a paranoid schizophrenic, and that can be treated with drugs and behavioral therapy.
"If, however, he had a personality disorder or was just ... motivated, as in this case, by a misguided political belief that this was the only way to stop the Islamization, as he would have it, of his nation, then in those circumstances he has be found guilty because he understood what he was doing was wrong."
Breivik's rampage, the worst atrocity on Norwegian soil since World War II, prompted much soul-searching.
Norwegians reasserted their commitment to multiculturalism and tolerance at a series of mass public tributes held in the immediate aftermath of the massacre.
And earlier this month, Norway's chief of police stepped down after an independent commission detailed a catalog of police and intelligence failures.
It concluded that those errors cost police 30 minutes in getting to Utoya, and that dozens of lives might have been saved.
Speaking last month on the anniversary of the killings, Prime Minister Jens Stoltenberg urged Norwegians to "honor the dead by celebrating life," and said Breivik had failed in his attempt to change Norway's values.