(CNN)-- During much of Nelson Mandela's imprisonment, communication with him was limited. In the early days on Robben Island he could only receive one visitor and one letter every six months.

violence was the only weapon that would destroy apartheid." Nelson Mandela, 1959 "Monitoring Mandela (Four Police Letters)" These letters capture what it was like to be a banned political activist.

In the winter of 1964, Nelson ... one letter every six months. But Robben Island became the crucible which transformed him. Through his intelligence, charm and dignified defiance, Mandela ...