Too bad kingdom rush was not called empire rush, but that name does make sense since it takes place in the late medieval times. But for kr4, if it was called empire rush, it could cover the early modern warfare(either 1700s or 1800s), since the most powerful nations were empires(correct me if im wrong). This would mean new weapons for the game, like muskets, bolt action rifles, machine guns, cannons, mitrailleuses,revolvers,trains!!! Or it can also cover the roman times. Gladii,Pilums,great shilds, balistas. These are probably only the 2 periods the name can cover since early modern warfare wont go too far from the fell of a great and powerful nation, and the romans actually had artillery. But hey, thats 2 more games of KR.
Roman army(not sure what period)
Jacobite rebelion(not sure which one)
Fredrick the great
Joseph 2 of austria
Charge of the light brigade,crimean war
Otto von bismark(not an empror)
Wilhelm 1
napoleon bonaparte
Napoleon 3
Abdülmecid 1