Minutemen or Militia are the immovable guards in Imperialism.
Minor nations usually start with three regiments of Minutemen in each non-capital province. Great powers usually start with four (though Russia in the Ottoman Empire 1820 scenario starts with eight). Four can often resist a weak attack by stronger but fewer units; for example, an attack using only one Regular, one Hussar, and one Light Artillery may fail and lose the Regular. Adding a second light artillery may still lose the battle but lose no units.