Update: I’ve written a strategy guide for Imperialism II at the 400 difficulty level.
You can win an Imperialism II game on 200 difficulty setting without a single new world province. In all honesty, I think there’s some kind of bug which allowed me to win, however.
For much of the game, Portugal’s fleet was hovering outside of my port. I had a colony in the New World, two provinces of the The Mayans. The Mayans were consistently able to sell me a diamond, a gold and a silver, and that kept my cash flow up enough to be able to take most of the provinces on my continent and buy spies to stay close on the research. I could not tell if the Portuguese fleet was blockading or patrolling, but it seemed like it should have prevented me from receiving the trade goods from my colony.
I carelessly lost my colony to Holland and had to rely completely on trade for cash from that point forward.
The map key for this game is Llantea.
Here was my final score: