I don’t believe the terms ‘base’ and ‘superstructure’ were articulated by Marx/Engels until later, but the gist of these distinctions is already apparent in the earlier work, The German Ideology. Reading the first part, on Feuerbach, one can’t fail to notice that while the critique of the young Hegelians is in itself generally cogent, the reasons for what appears to be a relatively simplistic conception of history, in which productive forces and relations are the real bases of the shadow play of ideas and conceptions (religion, politics, law, philosophy), are almost entirely lacking. Marx and Engels provide nothing other than rhetoric as to why we should swing vigorously from one pole, in which the self-unfolding of Spirit (or what have you) determines history, to the opposite pole, in which the development and revolutionizing of productive forces determines history.
The much more reasonable conclusion is of course that both are mutually dependent. And while later Marxists generally attempt to interpret Marx this way (and decry those who adopt a naive economic determinism as “vulgar Marxists”), there is little in The German Ideology to prevent the reader from reaching this vulgar conclusion.
If the economic base “gives rise to” or “conditions” the superstructure, then that may leave room for the superstructure to in turn exert causal influence on the base. But why make the base dominant at all? What historical or philosophical evidence is there for making the base the more fundamental of the two? It cannot be the case that a superstructure could not exist without a base, because by the same token an economic base could not exist without a superstructure (law, politics, mores, etc.). In the haste to undertake the worthy and necessary criticism of the (young) Hegelian over-emphasis on the role of the concept/idea/Spirit, early Marxism clearly swung too far in the opposite direction.