T3dlib1.obj : error LNK2001: unresolved external symbol "struct HWND__ * main_window_handle" (?main_window_handle@@3PAUHWND__@@A) Debug/Tile Engine.exe : fatal error LNK1120: 1 unresolved externals I searched through my whole project and removed all the main_window_handle''s in my project, then clean it and rebuilt everything and still the same error, whats wrong?

The linker is telling you that some variable was declared somewhere using "extern" and not defined. Either you forgot to include the library''s .lib (or .cpp) files into your project, or it expects you to define and create a main_window_handle which it will then use.

There should be more information about this in the book.