The Directory Structure Of Yii

Please help me, i have a problem with yii directory structure.

I have a website with backend (admin side) and frontend (end-user side), admin is uploading files and end user should access those files, the question is:

What is the perfect path for both backend and frontend to access those files? (common folder is good?)

Also i have a problem with Auth module, i need it to work in both sites so i put it in common/modules folder but i don’t know how to configure the path to make it accessiable from backend and frontend?