Question on Things Fall Apart

Even as Achebe works to educate his readers about African culture and to combat demeaning stereotypes, he does not present Igbo society as ideal or perfect. The portrait of this culture on the eve of its "falling apart" in Part I of Things Fall Apart for example, is complex, sometimes contradictory and critical. What aspects of pre-colonial Igbo culture does Achebe seem to question or criticize? How does Achebe use characters like Obierika, Okonkwo, and Nwoye to offer such social criticism of Igbo society? How do the people of Umuofia react to change?