1."I'm assuming that the viking_build_prod.txt and viking_unit_prod.txt are the ones used for the Viking campaign, while the "crusaders" files get used for the main campaign. projectilestats.txt is shared", correct? -yes.

Also in the campmap/names folder you get a separate viking heroes.txt, as well as separate files in the loc/eng folder as well.

2." For the farm upgrades, in the build_prod file they are listed with 200, 400, 480, 560 "income" in viking, and 120, 140, 160, 180 "income" in crusaders. These are supposed to be percentages, not actual income figures like the ones given for mines or cathedrals. Is this sorted out in the engine by the BuildingType label of FARM_INCOME? Anything special about LEV1_INCOME and CATHEDRAL_INCOME BuildingTypes?"

Yes, they are indeed percentages- the general formula being if base income level is 100, then the first upgrade will generate 120 in the main campaign and 200 in the viking campaign subject. This however is modified by a number of factors, so that actual income may vary from 110-124(only for illustration-not actual figures). Search the site for more detailed information.

In case of LEV1_INCOME and CATHEDRAL_INCOME you get about roughly 55%of the amount mentioned in the income column- ex. a figure of 400 will generate about 208 florins.

Also, any idea what makes the Forest Clearing indestructable?- most probably hardcoded as far as I know.

3."Never seen this, because I always try to stay pagan as the Vikings, but what happens to pagan shrines when the Vikings convert? Do they just go away/get destroyed?" -yes they are destroyed as in case of any religious building alien to the culture actually occupying the province. ex the abbeys in the british isles when raided by you.

4. see post no.2 for first part and you get money from destroying buildings relative to how much it costs- but the vikings get a special hard coded raiding bonus as well as the ability to raid without using ports.

5."I always thought that building the first-level castle building gave a happiness bonus, but I see that is not the case in build_prod. Is this just a figment of my imagination, or something hard-coded into the engine? I can imagine this being the case, since the castles get displayed on the main map. Any idea what (if any) the happiness bonus is?" -No happiness bonus afaik, but you do get a minimal chance of bandits appearing in a province with a fort- again hardcoded.

6. Again a formula exists somewhere- check in the giudes or post in the alchemist lab.