Networked cyber-physical systems are facing serious security threats from malicious attacks. It is noted that the networked cyber-physical system should take defense measures into account at the beginning of its construction. From the conservative defensive perspective, this paper proposes a robust optimal defense resource allocation strategy to reduce the maximum possible losses of the networked cyber-physical system caused by potential attacks. Then, based on the robust optimal allocation strategy, it can be proved that the topology of the networked cyber-physical system has a great influence on the loss function. In order to further improve security, the effects of adding redundant connections are investigated. Furthermore, by taking geographical knowledge into account, a hexagonal construction scheme is proposed for providing a geographically-feasible and economically-viable solution for building networked cyber-physical systems, where the loss function has a cubic decay.