It is true that, for Augustine (and this point of view is clearly antimanichaean), the Civitas Terrena is good by nature, and bad by vitiation of human will. The more we read his De Civitate Dei the more we find out the fact that he did not intend there to write a political theory but to develop his moral and religious considerations and perspects on human destiny. His point of view as a whole is extremely wide, deep in any aspect and always so historical, far from the so-called fideism. 
(1) Augustine says from his heart that human life is merely a distentio and he endeavours to recommend us to change this distentio into an intentio. This is the Leitmotiv of his lifetime. 
(2) Augustine contends to see the Historical World as the place of development of the Rationes Seminales. For this reason, the World History is fundamentally so much theological for him. 
(3) We should not seek any Tertium Quid between Civitas Dei and Civitas Terrena because our Lord did teach us to say only "yes, yes or no, no."