The word “cyber,” originating from the Greek word meaning “skilled steering or guidance,” has taken on the modern meaning of using digital communications within and among intelligent devices to perform information gathering and commanded control. Information technology (IT) systems include networked communications among computers, business systems, and the internet. Operational technology (OT) systems include networked communications among industrial control system (ICS) devices performing automatic safety, operational, and monitoring processes. In this paper, established ICS methods and standards are used to design defense-in-depth cybersecurity methods for digital communications within an energy control system (ECS) communications network. The ECS communications architecture is a mission-critical ICS and is divided into multiple levels with unique requirements and features from the process up through the station and finally to the control center. Using these levels, it is possible to identify interacting cyber defense technologies, the levels at which they should be deployed, and which devices they belong to (IEC 62443 Part 3) instead of the arbitrary defense-in-breadth strategy of requesting that every device include every cyber defense technology (IEC 62443 Part 4).