Hmm, I followed these steps, and the mod loads fine now:
- changed building_cost to gold_cost
- added new values to defines.lua
- added new values to opinion_modifiers
- removed graphical_culture_type from special troops
- added tribal to static_modifiers
- added in_hiding and winter_soldier to traits
- added 'seasons = "seasons.txt"' to default.map
- changed those bookmark names
- added buildings\tribal.txt
- added climate.txt
It crashes when I click on single player though, but multiplayer works fine.
Any ideas? I strongly suspect it's because of the new start-menu with the eras thing, which I haven't edited, besides using the vanilla bookmarks folder and just changing the date and description for the Fate of England bookmark...
Do you added seasons.txt in the map folder?