The biggest challenge for me was figuring out the dozens of functions in the "usecode". These do all the important functions in the game (like "add_party_member", "show_sprite"), and only a few were known when we started. Originally, I was only interested in supporting Black Gate, but then Ryan started figuring out the functions used in Serpent Isle (which were quite a bit more complicated). And once you get started on a project like this, it's hard to stop.