Long-range units like archers, musketmen, and cannons are essential for victory, but they're useless in close-range battle. Use sword fighters to keep them safe.
Win the optional EXP missions and you'll have points to upgrade units and buildings. Those bonuses will last for the rest of the campaign.
Move your infantry into adjacent rows if they're in line with ranged units. Walking headfirst into a barrage of arrows or musketballs isn't a very good strategy.
Place buildings near appropriate scenery to generate bonus resources. Make sure you tap the resource icons to collect them.