Icons of History Achievement in Age of Empires: Castle Siege

You can unlock heroes by building the Hall of Heroes in the 3rd age. You’re automatically granted your civilizations first hero upon completion of the building. To unlock/research the others, you need to earn 100 pennants from other civilizations which can be done by attacking cathedrals and monasteries. Once you earn 100, you can unlock them or save up for the second-tier heroes which will be available with the next upgrade of your Hall of Heroes.