Historic urban areas face both natural and anthropogenic risks, which can appear slowly (e.g. increasing temperatures, air pollution), or suddenly (e.g. earthquakes, terrorist acts). These risks can alter user behaviour and can imply the necessity to activate emergency procedures. Risk assessment and mitigation tasks should hence consider the interaction between multiple hazards affecting the built heritage and urban fabric, and by including the analysis on users’ behaviours. This assumption is particularly relevant in the context of historic public open spaces, such as squares, where the convergence of hazard and building heritage vulnerability could be further intensified by critical levels of users’ exposure and vulnerability. This work presents a multi-risk assessment and mitigation approach for squares, considering the presence of large crowds and simulating their response to slow and sudden emergencies. The methodology addresses the combined effects of increasing temperatures and terrorist acts using a probabilistic, multi-agent, and cellular automata-based model. The results are assessed using key performance indicators (KPIs), aimed at combining hazard, vulnerability, and exposure factors through expeditious and simulation-based methodologies. The approach is applied to Piazza dell’Odegitria, in the historic centre of Bari, Italy. Sustainable single and combined retrofitting solutions are proposed to reduce multi-risk by up to about 16%. These solutions are designed to be simple, compatible, reversible, and tailored to the square’s characteristics. The results highlight the benefit of the investigated methodology allowing local authorities to evaluate various environmental and spatiotemporal configurations as preliminary strategies for risk mitigation in historic urban areas.