It's possible to extract campaigns from the game itself and edit them like Custom Scenarios.

An unused Trade Workshop building can be found in the game's data. It was originally planned to function as a building where to player could assign villagers to produce "Luxury Good", which could be sold for gold. Though scrapped, remnants of their existence can still be found in the game's scenario editor, with its icon resembling the Market economic building, and it prior to the game's HD expansions would have the same design for all architectural styles except Eastern Asia. The building can also be seen in several scenarios, too.

Prior to deciding on what the new civilizations for the existing architectural styles would be (Spanish, Koreans and Huns) in the Conquerors Expansion of Age of Empires II: The Age of Kings, previously considered civilizations had been:

• Western European: Italians.
• Eastern Asian: Khmers and Tibetans.
• Central European: Swiss, Magyars, Slavs and Hasburgs.

In addition, the new architectural style was chosen to be for new world civilizations because the developers found the Aztecs to be cool, had name recognition, and had been defeated by a concatenation of lucky flukes.

Many of these scrapped civilization ideas would later be added into the HD Expansions of Age of Empires II.

During development, Wolves were meant to have much more complicated behavior. Every new match would have wolves start the game by forming a pack and then wandering across the map, seeking food sources while avoiding towns. Their level of aggression would vary based on their level of hunger. Furthermore, players would be able to hear howling in the background at points, the frequency of which would hint at the threat the wolves posed.

Despite being largely forgotten in favor of its sequels, the original Age of Empires remained popular decades later in Vietnam, due in large part to pirated copies of the game being bundled in Vietnamese computers and a long history of LAN matches dating back to the 1990's. Modern Vietnamese players often use custom rulesets, and top players have earned tens of thousands of dollars in tournaments viewed by audiences as high as the hundreds of thousands.

In 2023, this popularity in Vietnam was formally acknowledged by Forgotten Empires, the modern developers of the series, in their "Return of Rome" expansion for Age of Empires II, which ported and updated content from the original Age of Empires to Age of Empires II: Definitive Edition. The expansion included a new faction, Lac Viet, "in honor and recognition of ["The Rise of Rome" expansion from the first Age of Empires game's] phenomenal popularity in Vietnam."

Developer Sandy Petersen has given multiple explanations as to how the Koreans were added as a playable faction in the game's Conquerors expansion in 2000. On the Age of Kings Heaven forum in 2016, Petersen explained that the Koreans were chosen as one of a few candidate "Oriental" civilizations that could use the existing East Asian architecture art assets, along with the Khmers and Tibetans. He said that ultimately, attracting potential customers in South Korea was just one of several deciding factors.

Years later in 2025, he gave a different, contradictory account of events, stating that the Koreans were a last-minute addition handed down from Microsoft only five weeks before the project's completion. He also mentioned that one of the design decisions he made for Koreans was to give them free Archer armor upgrades. However, this bonus wasn't added to the game until Update 42848 in 2020, many years after Petersen's work on the game finished.

In the Age of Empires II competitive scene, a "Daut Castle" refers to a Castle that either requires the death of many of one's Villagers to complete, or is left unfinished (but nearly complete) due to an enemy attack. The term was coined in 2016 and named after Serbian competitive player DauT, who lost a large number of Villagers before having his Castle quickly destroyed in a broadcasted game. In the game's 2019 Definitive Edition, this moment was acknowledged through the achievement "Castle of Doubt", which is rewarded to players who lose an unfinished Castle that is 95% or more towards completion.

In the third scenario of the Genghis Khan campaign, titled "Into China", it is possible for the Mongol faction player to capture Bombard Cannons from the Chinese faction. However, in standard play, it is impossible for either civilization to build and access this unit.

In the original game, the scenario "The Battle of Stirling", which is based off of the historical Battle of Stirling Bridge, did not feature an actual bridge on the map, likely resulting in the abridged name of the scenario. The Stirling Bridge was eventually added to the map in the game's Definitive Edition, serving as a means for the Scots to cross over onto the English camp.