When Ngugi wa Thiong'o is talking about colonizers, he says the the real aim of colonialism was to control people's wealth; what they produced, how they produced it, and how it was distributed.
Colonialism imposed its total control over the social production of richness through military conquest and political dictatorship.
But the most important area of domination was the mental reality of the colonised; the control through culture, how people thought of themselves, and their relationship to the world.