Relating the megastructure to the issue of the Commons is a useful exercise to understand the success and the disappearance of what Peter Reyner Banham called the “dinosaurs of the Modern Movement”. All these large-scale constructions suffered the same fate: a conflict between the promise of a large shared space and the temptation of its fragmentation. This quantitative quandary is also raised in another field by Garrett Hardin in 1968 as the ‘enclosure dilemma’. The publication of his article “The Tragedy of the Commons” sparked a broad controversy coinciding with the megastructure’s momentum. By assessing a number of theoretical correspondences, the article reexamines the impact of megastructures on the interdisciplinary debates of the time. It also considers the relationship between architecture and property as one of the possible–and tragically coincident–reasons for their success and dissolution.