A remote tower center (RTC, also "remote tower centre") is a building where controllers work camera-and-sensor feeds for airports that may be hundreds of miles away. Each airport is safety-assessed and approved on its own terms.
Real examples: Sundsvall RTC (LFV, Sweden), the Leipzig RTC (DFS, Germany), the Bodø Remote Tower Centre (Avinor, Norway, one of the largest, scaling toward 15+ fields), and RTC Vantaa (Fintraffic, Finland, purpose-built for multi-airport operation).
Operating modes at an RTC range from single-airport positions to sequential and, where approved, simultaneous multiple operations. Consolidating positions is a major driver of the business case and of controller quality-of-life, but multiple operation carries its own workload and safety considerations.