Guidelines, hints/tips for test design at L3

LevelStructural Integrity
ObjectiveTo ensure that the internal code structure is clean
What to testEUT at this level can be a any structural component like a class, function, API, service, re-usable design components etc.
Test for whatTypical issues looking for at this level are code/structure/design related issues. Issues like concurrency handling, timeout issues, exception handling, resource usage policy, data usage conditions, linkage of components etc will be targeted at this level.
Design guidelinesHere we are designing scenarios/cases and execute to find out these issues in the code, rather think of possibility of such issues while coding itself and prevent them initially. Typically we can have a checklist of Potential issues at code level and ensure the code is handled such issues during development itself by any static analysis method.
Hints/TipsAt this level, typically no need to write design scenarios/cases explicitly. Based on the code design, language used to develop the code, identify the set of possible issues and ensure such issues are prevented/taken care by the code. No need to execute the test case and prove that the issues do not exist, rather prevent it while coding itself.