unity-asmdef

Start by splitting editor code from runtime code, then define a few meaningful assemblies per domain rather than many tiny ones. Keep the dependency graph directional and shallow, and isolate test assemblies so they only reference what they verify.

Assembly definitions are worth introducing when the project has multiple domains or systems, editor and runtime code are mixed, compile times become noticeable, or tests need clean isolation. For tiny prototypes, asmdef fragmentation adds overhead without benefit.

Assembly definitions split code into separate compilation units, so Unity only recompiles assemblies whose dependencies changed. This reduces iteration time in large projects compared to the default single Assembly-CSharp compilation.

Yes, circular references between assemblies are not allowed and will produce compile errors. Avoid them by keeping the dependency graph directional and never forcing everything through a shared dumping-ground assembly.

Over-fragmentation creates maintenance churn, complicated reference management, and harder refactoring across boundaries. Prefer a few meaningful assemblies aligned with real domains over many tiny ones.