In this chapter, we have considered tasks that touch several layers of the MVC architecture. First, we have seen how to cover extensions for multi-languages by adjusting static text in the templates and also country-specific formats like date or time are adjusted. Also, we have shown that also the domain model itself can be translated. After this, we show the check of invariants in the model, including displaying error messages in the template and the possibility to correct wrong entries by the user. At last, we have shown concepts that apply to the security of the extension. Important by this is that you have understood what request hashes are and how to prevent cross site scripting.