Moving from the premise that future Moon and Mars settlements shall not take place in a lawless space, this paper addresses the question of how to develop and manage them in accordance with international space law. To this end, it conducts a systematic analysis of the Outer Space Treaty (OST), with the goal of assessing the scope of the freedom to use celestial bodies under international space law. Based on this analysis, and building on the successful experience of the International Space Station, the paper proposes the development of open international settlements made of shared modular facilities. In accordance with the principles of adaptive governance and subsidiarity, the paper argues that the regulation of such settlements should be based on a multi-level framework integrating international recommendations and bilateral arrangements. Under the proposed governance scheme, international recommendations should provide a general framework enabling the development of the settlement, while leaving its management to the mutual agreement of the parties.