Maps are dotted with (trains or wagons that provide resources or XP) and Native Settlement sites. By building a Trading Post on a Native Settlement, you unlock that tribe’s unique units and technologies. This creates a "land grab" dynamic early in the game—controlling the Mississippi River or the Silk Road is often more important than controlling a gold mine.
Moving the setting to the Colonial era forced Ensemble Studios to rethink combat. The rock-paper-scissors dynamic of infantry-cavalry-archers was evolved to fit gunpowder warfare. The game emphasizes the concept of "counters" more aggressively than its predecessors. Musketeers are powerful generalists but fall to Skirmishers (light infantry); Skirmishers are fragile but decimate heavy infantry from a distance; Hussars (cavalry) can run down Skirmishers but are vulnerable to Pikemen or Dragoons. Age of Empires III