The Great Siege of Malta had lasted a hundred years, or so the legends said. In the world of Age of Empires III, the limits of reality had been shattered by the "Unlimited Population" mod. No longer restricted by the 200-cap mandate of the Old Gods, the Ottoman Empire and the Spanish Armada had expanded until the very earth of the New World groaned under the weight of their boots.

A classic for the legacy version that gives all civilizations (including AI) infinite population and removes building restrictions. 230 Pop Limit Mod (DE):

• Performance Issues (The Big One): AoE3 was released in 2005. The Definitive Edition (2020) is better optimized, but the engine still struggles. Once you cross ~800-1000 units on screen in a multiplayer match, the game turns into a slideshow. Pathfinding, in particular, becomes nightmarish.
• Game Balance Is Destroyed: This isn't a bug; it's a feature. But know what you're getting into. Civs with cheap, spammable units (Russia, Ottomans) become god-tier. Expensive, elite-unit civs (Spanish, Dutch) suffer because their units cost a lot of resources and pop. In unlimited, pop cost doesn't matter, so only resource cost matters. Suddenly, the British Manor House boom is unstoppable.
• The AI Doesn't Know What to Do: The standard AI has hard-coded build limits. It will build a "large" army, stop, attack, rebuild. With unlimited pop, it might just build 500 villagers and then sit there, confused. Or it will crash.

Background

How to Install the Age of Empires 3 Unlimited Population Mod

With a trembling hand, he struck the seal. The world didn't end. Instead, it grew quiet.