The COSMOS platform's technical architecture is designed to meet both quantitative and qualitative requirements associated with a fully programmable city-scale shared multiuser advanced wireless testbed facility. In addition to realizing Gbps+ radio speeds with low latency, the design should support a number of capabilities including remote multiuser access, virtualization of resources, open programmability, flexible topology, node mobility, diversity of radio environments, reproducibility, usability, instrumentation, extensibility and interoperability. Perhaps the most important design consideration is that of open APIs and full programmability across all the technology components and protocol layers. Our approach to realizing full programmability is based on a multi-level SDR and network architecture in which signal and protocol processing functions can be flexibly placed at radio nodes, edge cloud resources or general purpose cloud servers depending on the desired functionality. The testbed’s focus on ultra-high bandwidth wireless implies the need for significant SDR computing capability in the radio access network – rather than placing all the compute functionality at the radio node, having a second layer of "cloud RAN" capability makes it possible to offload a significant part of the node's function to an infrastructure-based computing cluster. The same edge cloud can be used for network and application level processing particularly in scenarios requiring low latency end to-end response.