This paper examines the evolution of Tokyo and Osaka from historical castle towns into modern mega-agglomerations. It reviews academic literature on the economic, social, political and demographic factors that drove the rapid expansion of these cities, leading to high population densities and intensified urbanization effects. The article utilizes historical population data, spatial analysis of urban morphology changes, and recent statistical data on the Tokyo and Osaka metropolitan economies. It is found that strategic location, industrialization, transportation infrastructure, rural-urban migration, suburbanization and urban consolidation policies all contributed to Tokyo and Osaka's transformation into megacities. The paper also compares and contrasts the development trajectories, geographic constraints, and agglomeration profiles of the two cities. It concludes by assessing the challenges posed by further megacity expansion in Japan.