In the course of my current mod I discovered the following: if you add new buildings to the build_prod file they MUST go at the end to avoid corrupting save games. It seems the save game must index building types rather than have any "real" info about them. The reason I concluded this is that I added some extra buildings, but halfway down the list. New campaign was fine everything worked wonderfully (he said, modestly ) but when I tried a save game, all the buildings had altered, including things like spearmaker and spearmaker's workshop both appearing in the same province (I was trying out metalsmiths being divided into two separate building lines for bronze and iron technology) and it seems like the spearmaker got transposed into the metalsmith slot. ie the save games probably record something like "Building one, level two" rather than "Spearmaker's workshop". It's a wonder it didn't crash completely, but I reckon it's another little insight that may come in handy...