Geographic digital twins (GDT) are increasingly used as place-based platforms to support monitoring of environmental and social processes underpinning social innovations. This research addresses a specific methodological gap: whether a modular GDT architecture can sustain consistent deployment across five distinct European Transition Coastal Labs (TCLs). Three bottlenecks shaped the research: baseline datasets were held by different organisations and were rarely maintained as a shared reference; real-time sensor observations were managed within IoT systems separate from contextual GIS layers; and participatory inputs remained disconnected from mapped evidence. EmpowerCoast was designed and deployed within the Horizon Europe EmpowerUs project across Connemara (Ireland), Træna (Norway), Cap de Creus (Spain), Limassol (Cyprus) and Åland Islands (Finland). The platform implements a modular, OGC-compliant full-stack architecture integrating spatial storage, microservices, RESTful APIs, 3D visualisation and vendor cloud IoT ingestion. Four deployed proof-of-concepts are reported: near-real-time solar irradiance monitoring in Connemara, near-real-time chlorophyll-a monitoring in Træna, participatory storytelling and citizen science across all five TCLs, and ingestion of harmonised EMODnet habitat layers for Cap de Creus and the Åland Islands. Analysis of the deployed codebase, within a single consortium deployment, shows that consistency was located in exchange format, delivery protocol and structural pattern rather than in shared instances: sensor observations from two unrelated commercial ecosystems, differing in authentication, payload schema and measured parameter, produced structurally identical platform behaviour. By demonstrating that architectural consistency survives vendor, domain and jurisdictional heterogeneity, the UN Ocean Decade endorsed EmpowerCoast platform enables coastal communities across Europe’s Arctic, Atlantic, Mediterranean and Baltic shores to observe and interpret environmental changes through a shared spatial infrastructure.