If I was to inherit a large codebase, how would I start refactor the application using TDD?
The application has layers of authentication(financial banking company), and frankly, not knowing where to start is a big issue.
I guess follow up is, is it worth the time and effort to start using TDD if the code is not "testable"?
This book is exactly what you need: Working Effectively with Legacy Code by Michael Feathers.
That book goes directly to both of your questions: How to start refactoring a legacy codebase, and how to approach TDD when the code is not yet testable.
Underlying both of those questions is another that the book answers: How to get a legacy codebase under test.
