No Subsystems

From a certain size on, a system’s structure – if it is defined exclusively on the package level – will become increasingly incomprehensible. If the system consists of more than 100 packages, for example, it is extremely difficult to recognize and define the structure between the packages and to maintain it consistently.

Reference

M. Lippert, S. Roock, “Refactoring in Large Software Projects: Performing Complex Restructurings Successfully”. John Wiley and Sons, 2006.