Industrial Control Systems (ICS) play a critical role in controlling industrial
processes. Wide use of modern IT technologies enables cyber attacks to
disrupt the operation of ICS. Advanced Persistent Threats (APT) are the most
threatening attacks to ICS due to their long persistence and destructive cyberphysical
effects to ICS. This paper considers a simulation of attackers and defenders
of an ICS, where the defender must consider the cost-efficiency of implementing
defensive measures within the system in order to create an optimal
defense. The aim is to identify the appropriate deployment of a specific defensive
strategy, such as defense-in-depth or critical component defense. The problem
is represented as a strategic competitive optimisation problem, which is solved
using a co-evolutionary particle swarm optimisation algorithm. Through the development
of optimal defense strategy, it is possible to identify when each specific
defensive strategies is most appropriate; where the optimal defensive strategy depends
on the resources available and the relative effectiveness of those resources.