We propose a method to build critical embedded control systems in a systematic way. The method covers the modelling of both the digital part and the physical environment of a considered system, and their refinement until more concrete levels. It is based on Event-B in order to benefit from its materials, step-wise refinements and tools. Two main processes are distinguished: one to capture the global model, the other to detail the global model; they are made of several refinement steps which are accompanied with guidelines. The precise description of the interface between the digital and physical parts is used to start the modelling process. The recurrent categories of variables and events in control systems are described and used as guidelines to conduct a systematic construction. We illustrate the method with the landing gear system case study.