I want to create a simple stacking window manager (in C) for private use, mainly for the purpose of learning and challenging myself.
I've looked through twm's source code which has relatively few bells and whistles but it seems very low level since it's not based on a widget toolkit.1 Would using a toolkit such as GTK+ be preferable? I'm afraid that some of the code and libraries in twm might be too antiquated (edit: deprecated) and I want the window manager to use relatively modern libraries. For the sake of understanding I would also be interested in suggestions how to start a window manager from scratch — there aren't many tutorials for this purpose.
Update: For those thinking of similar projects: I ended up using Common Lisp and the CLX library. tinywm-lisp served as a basis and the brilliant CLFSWM and Stumpwm were a great help. For reference I used the CLX — Common LISP X Interface (PDF warning) and #xlib on Freenode.
Whatever you do, use XCB and not Xlib. It' modern, asynchronous, simpler and gives you direct access to the X11 protocol.