The court was told that as they approached the lift, the boy noticed a waterpark outside.
It was said that Foster pushed the lift button, placed the child on the windowsill and the “inconceivable” unfolded before he could react.
Alexandrou told an earlier hearing, which took place on October 6, that Foster’s life “changed forever” from that moment.
Police were called to reports of a child falling “under unknown circumstances” at about 18:40.
The boy was taken to Paphos General Hospital and later pronounced dead.
In a voluntary statement to police, Foster said when he placed his son on the windowsill, the boy “fell before he had a chance to grab him”.
While Foster had alcohol in his system, it did not form a significant part of the case against him – and his lawyer said there was no testimony he was drunk.
Alexandrou said Foster was held in custody for eight days immediately after the incident and had to call his daughter from custody to explain what had happened.
The court heard the window was 2.85m (9.3ft) wide and 1.2m (3.9ft) high.