This book focuses on understanding how a megacity like Seoul can be read as a formal architectural composition and not an endless urban sprawl. In a broader sense, the book discusses the dichotomy between city and urbanization: “city” being an architectural problem of bounded forms, while “urbanism” is an infrastructural project of expansion. It is an uncontested reality that urbanization is a continuous global process that has produced nebulous conurbations labeled as megacities. These expand beyond the virtual administrative boundary of any said “city,” producing a discrepancy between an area of administrative control and the real physical condition of human settlement. If there were a better formal understanding of megacities through their typological architectural conditions, then there could be a better assessment of the qualitative state of urbanization. Avant-garde groups from the 1950s, 1960s, and 1970s such as Team X, the Situationist, the Structuralist, and the Metabolist worked with ideas of megaforms and megastructures to address this issue. Although most of these proposals remained as paper architecture, this book reevaluates some of these ideas for the 21st-century megacity, using Seoul as a case study due to its clear typological formations produced over its diff erent periods of governance. The aim is to present the concept for an infra-architectural hybrid model of typological islands and subterranean megastructure that organizes Seoul as a fl exible multi-linear city. This book will be of interest to academics and students of architecture, urban geography, and Asian studies.