A design environment for security critical database applications that should be implemented by using multilevel technology is proposed. For this purpose, the Entity-Relationship model is extended to capture security semantics. Important security semantics are defined and a language to express them in an ER model by means of security constraints is developed. The main contribution consists of the development and implementation of a rule-based system with which security semantics specified may be checked for conflicting constraints. The check involves application independent as well as application dependent integrity constraints and leads to a non conflicting conceptual representation of the security semantics of a multilevel secure database application.