RE: Improvement selection:
I haven't dug around in the XML code for that (and I'm not sure where to look to be honest) but Fall From Heaven's moved Lumbermills to Archery, which is what I did when I modded Fall Further, and the AI really seem to like Lumbermills. (I can't blame them, they're pretty good that early.)
I'm against giving castles any sort of buff, as opposed to just finding some way to make the AI use them correctly. Since I don't think that can be done, I think despite being a Vanilla Civ IV improvement we should use the scale we do for our own things - does it do more damage than it adds for good? Nobody I know ever builds the forts themselves, and I think the stupidity of the AI makes it a better idea to just avoid having them in the mod outright.