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Ugrade your cav tech as first priority then inf and art and then navy/forts+barriers etc. You don't really have to upgrade navy tech until the british campaign. As for generals, get cav gens with auras first. Cav gens are OP in this game. So I would recommend you major in cavalry, and once you get  1 or 2 cav gens (murat, ney or dabrowski) get an infantry or art gen (try to get Aura gens like Karl, Massena, Mahmud, alexander). I wouldn't get a navy gen until late game, as the navy campaign missions can be done by high navy tech ( I think there are only 4 pure navy missions - 1 in the russian campaign, 3 in British campaign, and 6 from hood challenges, excluding crimean war challenges). But get a cav gen as your first general, and buy healling items (ambulance, tents) and get the best item for each unit (flintlock pistol for cav, artillery telescope for art, etc.), and then move on from there. Farm 1798 netherlands if you need medals- Its the most efficient source of income as you get 50 medals in 15 ish mins and also 10,000 coins which you could use to upgrade tech levels. Also, don't buy any of the coin items in the shop- they're useless, and you get many of those for free from campaign achievements.