The resilience of the urban outdoor built environment to terrorist acts depends on the interactions among the physical scenario, the attackers, the hosted users, and the mitigation solutions (both structural and non-structural), when implemented. Due to the complexity of the system, expert risk assessment methods should be also supported by simulation-based approaches. In this sense, this chapter first proposes a method to jointly consider hazard, vulnerability, and exposure in outdoor Open Areas (OAs) by then identifying possible emerging typologies and points of attack. Then, the behavioural design approach is used to evaluate the impact of different input conditions on final risk levels depending on the users’ response to the terrorist act. In this sense, the quantification of user exposure and individual vulnerability is provided, since these parameters can vary over time and space, offering a complete view of input scenarios in case of terrorist act in the OAs. Then, the simulation of user behaviours in such defined emergency and evacuation scenarios can be performed thanks to experimental-based models. Key performance indicators (KPIs) are proposed herein to organize simulation results and quantitatively derive the risk levels in the built environment. Finally, regulation-based mitigation and protective strategies are identified, by considering implementation issues, but their effectiveness could be assessed by using the proposed behavioural-design-based methods taking advantage of simulation about the emergency and evacuation process.