Anders Behring Breivik's defence have told his trial he should be considered sane and have sought his acquittal - a request regarded as a formality.
Lawyer Geir Lippestad told the court in Norway his client had been driven by extreme politics, not violence.
The prosecution has called for him to be considered insane.
Breivik, who admits he killed 77 people and injured 242 on 22 July, is due to address the court himself on the last scheduled day of the trial.
The request for acquittal is a formality because Breivik does not accept the charges of terrorism and premeditated murder against him.
He argues that his attacks were necessary to stop the "Islamisation" of Norway, an argument outlined by his defence on Friday.
Breivik bombed government buildings in Oslo before shooting young Labour Party supporters at a camp on the island of Utoeya.
Memorials to Breivik's victims will be built at the two attack sites, the government announced on Friday.
Several people injured or bereaved by Breivik are also due to address the court on Friday.
A support group for his victims is reported to be planning to walk out of the courtroom when Breivik speaks.
It was, Mr Lippestad stressed, for the court to decide whether his client had been sane at the time of the attacks.
"The mother of these actions is not violence, it is an extreme, radical, political attitude, and his actions must be perceived from the point of view of right-wing extremist culture," he said.
The fact that "safe, little Norway would be hit by such a terror attack is almost impossible to understand", the lawyer said.
This, he suggested, could explain in part why psychiatric experts had reached different conclusions about Breivik's mental state.
He described his client as an ordinary young man with good friends and colleagues. How, he asked, would a man who was mentally ill have been allowed to join a shooting club?
Nothing in Breivik's life up until the "inferno of violence" on 22 July had indicated he was a violent person, the lawyer argued.
Had violence, not politics, been his main driving force, he could have gone to an Oslo shopping mall, Mr Lippestad said.
The prosecution took the stand again on Friday in response to Mr Lippestad, saying they were not convinced Breivik was psychotic.
However, there was enough doubt for the prosecution to ask for Breivik to be found unaccountable for his actions, they said.
On Thursday, in their closing arguments, the prosecution called for the killer to be placed in "compulsory psychiatric care", and not sent to prison.
It was worse, they argued, to sentence a psychotic person to prison than to place a non-psychotic person in psychiatric care.
As the lawyer spoke, Breivik sat calmly, occasionally sipping water.
Directly behind him sat several of those he had tried to kill on Utoeya while others were elsewhere in the courtroom, our correspondent reports.
Trond Blattmann, leader of the 22 July Support Group, told Reuters news agency: "For me the most important thing is that he [Breivik] is not going to be in Norwegian society anymore."