Smart cities have frontline responsibility to ensure a secure and safe physical and digital ecosystem promoting cohesive and sustainable urban development for the well being of EU citizens. S4AllCities integrates advanced technological and organizational solutions in a market oriented unified Cyber – Physical Security Management framework, aiming at raising the resilience of cities’ infrastructures, services, ICT systems, IoT and fostering intelligence and information sharing among city’s security stakeholders. Three pilot cases, with the engagement of a total of 5 cities in 4 countries (Spain, Romania, Czech Republic, Greece), enable S4ALLCITIES System of Systems to deploy and validate its intelligent components and functionalities on actual environment, ensuring the delivery of solutions and services in line with smart cities emerging requirements, focused on: risk-based open smart spaces security management; cyber security shielding; suspicious activity and behaviour tracking; identification of unattended objects; real-time estimation of cyber-physical risks in multiple locations and measures activation for effective crisis management. S4ALLCities smart components, integrated within 3 Digital Twin Systems, will demonstrate their technological advances in tackling terrorist attacks with high risk for mass casualties, within the complex environment of open crowded spaces. The multidisciplinary consortium (9 EU countries) will exploit project’s innovative technological systems in the global market providing a cost efficient and market ready integrated solution. S4ALLCities exploitation phase will promote good practices and guidance material across EU cities so as to enhance capacity building of involved stakeholders, reduce the vulnerabilities of public spaces, mitigate the consequences of adversary attacks, raise public awareness and strike a balance between improving security and preserving the open nature of public spaces as well as citizens’ sense of freedom.