A structured approach is key to solving complex problems in systems design. It acts like a magnifying glass, highlighting hidden details and patterns that might go unnoticed. Over the past few years, I have applied and refined this approach while designing and extending ATC simulators and operational Air Traffic Management Systems (ATMS) for Dutch air traffic control. In this talk, I will share the lessons I have learned - successes and failures - in applying systems design principles within the ATMS domain.