The project is making good progress these days, though it was hard for me during the last few months to find much time to work on Solarus, for professional reasons (I am finishing my phd thesis) and personal ones (I just got married!).
If you are interested in how a quest is built, I just published some detailed documentation about the different data files that compose a quest. This documentation specifies the format of each file:
Hi everyone,
The blog is under construction and new pages may become available at any time. Today I am starting the technical documentation section of the engine (Solarus), which will provide a detailed specification of the engine and the quests management. Many subsections will come soon with the specification of each component of a quest (maps, tilesets, sprites, scripts, etc.). For now, you can browse the Doxygen documentation of the C++ code if you are interested in how the engine source code is designed.