As cities experience the digital transformation of the information era, informed decision-making becomes a first-hand necessity. Urban Digital Twins (UDTs), as the natural evolution of the smart city, are digital replicas of urban environments that model their structural and functional elements. However, their implementation is often hindered by technical complexity, data requirements, and resource demands. This paper proposes a globally applicable framework for prototyping UDTs by leveraging open-source software and global open data. Based on recent UDT implementations, essential system components and fundamental data typologies are identified to establish a minimal baseline for UDT prototypes. From the identified system components, a novel modular and automated UDT prototyping platform, UrbanDT, is developed and documented, integrating processing capabilities, data management, and interactive 2D and 3D visualisation. A fully automated workflow is presented for assembling UDT prototypes, encompassing baseline data acquisition, processing, and system configuration. The framework is validated through a UDT prototype for Bologna, Italy, demonstrating its applicability for data exploration and mobility-related street network analyses. Results indicate that the proposed framework enables reproducible and extensible UDT prototyping, offering a practical approach for cities and researchers to explore UDT concepts before committing to full-scale implementations.