The central theme of this book is the double role of the Sahara: as both a link between tropical Africa and the Mediterranean Islamic world; and as a restrictive/protective barrier that allowed considerable autonomy and originality to the southern Sahel ('shore') of the desert and its immediate hinterland. Thus Austen discusses the structural constraints upon caravan (as opposed to oceanic) long-distance trade but also show how this allowed considerable economic development to occur in the Sudan. Statecraft, urbanization, and literacy also remained limited by world standards of the medieval and early-modern eras, but a society and culture emerged that reflects a creative dialogue between the Mediterranean (global) and the African (local). Linkages to the Atlantic world during the first era of European expansion (extending up to the end of the 19th century for this region) intensified rather than undermined this process, but formal colonial rule, which substituted mechanized transport to the oceanic coast for caravan routes, finally marginalized the Sahara and the Sudan.