These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
In the Barbarossa campaign's last scenario, The Emperor Sleeping, after fighting your way through Damascus and the Saracen's camp, all spawning powerful units at will, you end up in a fortified but soldierless Jerusalem.
Better Than Canon: An extensive community exists towards creating custom campaigns using the in game scenario editor, to say that some of these have such effort and commitment put into them as to shame the official campaigns as released is an understatement. They can be found here.
Complacent Gaming Syndrome: Expect to see Deathmatch Hun only matches, and be criticized if you don't play Deathmatches as the Huns.
The very popular Castle Blood Arena (CBA), there are only 5 non-faction specific units worth producing in late game. Arbalests, Heavy Horse Archers, Hand Cannoneers, Paladins, and Heavy Camels. Although Pikeman can be situationally useful if fighting the Persians. The only reason to produce anything else is if you can't build any of those 5 units.
The final scenario for the tutorial campaign (the Battle of Falkirk) portrays it as a siege, and implies that William Wallace survives to continue his war with the English. While they were probably trying to avoid the historical Downer Ending, they could've just as easily given Wallace an Heroic Sacrifice.
In The Conquerors:
The religious building for the Aztecs and Mayans are still called Monasteries, and the religious unit is still called "Monk". However, the Aztec/Mayan monk actually uses a different skin than other monks, making it look like a Mayincatec priest. At least they got the blood on the stairs.
Also in The Conquerors:
Also, in the El Cid campaign, El Cid was said to have been killed by an arrow and propped up on a horse in order to stop the truth from spreading. In reality, he died peacefully. This was probably done for the same or similar reasons as the Battle of Falkirk bit.
Ensemble Darkhorse: For a very minor role in the first Genghis Khan level, Ornlu has gotten quite poular among the fanbase, enough for the developers to reference him in Age Of Mythology and Age of Empires III.
The Huns in The Conquerors don't require houses like everyone else. This means that, in Deathmatch (when you start with a huge stockpile of resources), the tactic of "Build a few Barracks, spam-click build Militia Button, and swarm everyone" ends up being this.
The Goths' ability to Zerg Rush. They get two unique techs, one that lets them build their unique unit, the Huskarl, at Barracks note Instead of Castles, and the heavy investments in Stone, villagers, and build-time that Castles require. Barracks can be produced quickly with cheap materials and only 1 or 2 villagers, and another that massively boosts the speed at which Barracks produce units. Combine those with the rather hefty Conscription boost (further reduction of build-time for units) and several Barracks, and you've got hordes of Huskarls, which are designed to tank arrows, and get an attack bonus vs. buildings. This results in an army that can shrug off towers, town centers, and even castles — and is replenished at lightning speed.
The eventually patched-out Teuton Town Center range boost.
The Persians' War Elephants, they're slow, but so powerful that outside of units they specifically weak against (monks, pikemen, siege weapons, which any reasonable army will have ways of dealing with), they're virtually unstoppable.
The various Korean onager bonuses allow them to outrange near enough anything, and the onager's obscene firepower and area of effect damage means they will cause insane damage before anything can retaliate.
The Korean war wagons also count, though seeming unlikely at first. Building one war wagon is a waste: expensive, slower and less ranged than a cavalry archer, doesn't make as clear damage as one or a scorpion and is easily overrun by a cavalry charge and destroyed. But mass enough war wagons, fully upgraded, and no enemy units will be able to make a dent on them.
British Longbowmen, once fully upgraded, can outrange any Castle except for a Teutonic Castle (a Castle's maximum range is 10-11 depending on the civilisation, the fully upgraded British Longbowman has 12 range, Teutons have 13). They can outrange any siege unit except for Bombard Cannons and Trebuchets (Trebuchets are damn slow and can't hit moving targets very well, Bombard Cannons barely manage to outrange British Longbowmen and are slower, and not everyone can make them); they can even match, if not outrange, the aforementioned Korean Siege Onager. Suppose they are near the sea, and the enemy decides to use their navy to destroy the Longbowmen? Well, the Longbowmen can outrange any ship (except for the Cannon Galleon, which is also pretty damn inaccurate (unless you're Spanish). They don't even need to worry about cavalry, because large numbers of them can shoot down entire hordes before they can do any serious damage.
The Spanish in late game seem to excel at practically everything, with no flaws in infantry, ships, cavalry, monks and fortifications, and bonuses on gunpowder, trade, blacksmith technologies and villager attack. They even get two unique units: the only gunpowder cavalry unit and the only monk cavalry unit in the game. They have bad archers that make them weak in the early game, but if you let them get to the Castle Age and beyond you'll get screwed. Guaranteed.
Good Bad Bugs: In the fourth Attila the Hun mission, you are tasked with destroying three major cities. Destroying the third spawns the massive Roman Army, a force of over a hundred top-tier units...unless you found and killed the placeholder unit on the map, which causes the army to be instantly defeated.
In the fifth Saladin mission, Jihad!, one of the three cities, Ascalon, is intended to attempt a Wonder victory. However, sometimes the AI for Ascalon bugs out and doesn't do anything. Given as how Jihad! is That One Level even without Ascalon going for the Wonder, this is immensely helpful.
The in game scenario editor can do some interesting things with structures and terrain elements that shouldn't be possible. This has been abused thoroughly by the modding community creating custom campaigns.
The Siege of Paris in the Joan of Arc campaign on any difficulty level other than "Standard": the English now have fully developed and highly aggressive Longbowmen that can snipe you dead from far away or lead you to their towers and castle's range. Once inside the city the simpliest mistake will have you swarmed with all the possible enemy troops in the city, including the above mentioned Longbowmen, Champions, Halberdiers and Siege Onagers. Even if you manage to evade all these foes, there's still the matter of leading Joan and at least 6 out of 10 villagers through a huge Burgundy attack safe and sound.
The most difficult part of this escort mission is there are NO HEALERS to support your forces.
The Saladin campaign is by far the most difficult despite being numerically ordered as the third campaign in Age Of Kings; made worse that the Saracens run on Magikarp Power in this game.
Lord of Arabia, the second scenario in the Saladin campaign. The level seems designed to screw you over in every way possible. You can't advance to the Imperial Age, start out with next to no resources and almost no units, and your opponents (especially Reynald's Pirates) can do everything you can't. Also, you get the privilege of defending two towns, Medina and Aqaba, who do virtually nothing for you and constantly nag you about crusaders attacking their caravans.
Jihad!, the second to last mission in the Saladin campaign has you up against Tiberias, Tyre, and Ascalon. You start out with very few resources, stuck in the center of the map, with Tiberias to your east, Tyre to the northwest across the ocean, and Ascalon to the south. Tyre and Tiberias will waste no time in hammering at your little fort, while Ascalon goes for a Wonder victory. Thankfully, the AI for Ascalon occasionally glitches and doesn't take any action, but it's still tough holding off two powerful opponents on both sides.
The Lion and the Demon, the last mission (again) in the Saladin campaign. Feeling good at building a huge wonder in Acre while repelling the continous assaults of Genoese (warships), Templars (rams and teutonic Knights), French (bombard cannons and hand cannoneers), Jerusalem (champions, camels and trebuchets) and Richard the Lionhearted, who can spam you with longbowmen and trebuchets AND has two unique trebuchets with crazily boosted range. Oh, and you'll have to defend the city from both north and west. Have fun.
On occasion, the AI might bug out after the initial assault and you'll get attacked by just Genoa with the other sides just hanging back and letting you win. However, that's the only way the mission becomes easy.
Into China, the third scenario from the Genghis Khan campaign. It pits you up against four Chinese factions: Tanguts, Hsi Hsia, Jin, and Sung. It would be a fairly standard scenario, if it wasn't for the fact that Jin starts building a wonder after not too long, located within its Imperial Age-level stronghold across the sea, forcing you to rush to the Imperial Age, while trying to scrounge enough resources to raise a force strong enough to punch through Jin's stronghold and destroy the wonder.
Barbarossa's third mission, Pope and Antipope, is like hell after the first two warm-up missions. The player must convert the Cathedral at Milan. While getting started isn't that hard, accomplishing the actual objective is. The player will be the target of frequent naval invasions, and even if one's defences are sufficient to ward them off, one still has to cross that river and fight Milan... and Milan not only has the resources to spam out troops at a horrendous rate, they also seem to have a fairly strong AI that makes them quite efficient at keeping their side of the river clean.
Just for that extra little irritation, the level starts you off with no villagers and a handful of monks, forcing you off to try to steal a villager from one of the opposing factions.
Barbarossa 4, The Lombard League, can be quite the nightmare as well, what with getting chased out of your base right off the bat and having to relocate to one of two places, neither of which has enough resources for you to beat all of your four enemies who attack you from both land and sea. Oh, and there's also the mission objective, which stipulates that you must build an obscenely expensive Wonder inside an enemy base. You know, those bases that are busy turning out soldiers faster than you can kill them and all that. Those bases. And there's one more detail that makes things more complicated than they really need to be: Henry the Lion betrays you again. No surprise there as if you needed another reason to hate him already.
The fourth mission in the Attila the Hun campaign. You have three enemies: Burgundy, who is weakly defended and can be persuaded to join your side; Metz, a fairly well-defended town; Orleans, a massive stronghold that will frequently launch powerful attacks at your position. Even worse, once you finally destroy the center of Orleans, the Roman Army spawns, a massive force of top-tier units that can completely turn the tide of battle if you aren't prepared. Unless you can find the placeholder unit and break the script.
Genghis Khan 4, The Horde Rides West, and Barbarossa 6, The Emperor Sleeping, are also quite hard if played with the expansion due to not being rebalanced to account for the Persian unique technology, which makes their elephants faster. Let's hope you can micromanage your monks.
The sixth mission in the Attila the Hun campaign. You have four enemies, all of whom try to defeat you with a Wonder victory, all of whom attack you with forces that are difficult to fend off, and all of whom are in fortified cities.
In the fifth mission of the Attila the Hun campaign, you are told in the briefing that the scenario will play out similarly to a multiplayer death match. God help you if you're not good at rushing. You will be attacked frequently and mercilessly by hordes of unit spamming.
Unfortunate Implications: By researching Atheism, the spy/treason tech cost (which allows you to see all your enemies on the map) is halvened. It seems that not being religious makes you a filthy traitor who would sell his country for gold.
Or you become an atheist but it's actually one hypothetical guy in your enemy's side who defects to yours and sells his country for gold. So if anything, in the game being an atheist makes you less squeamish about accepting help from people with dubious morals - according to the game, that is.
If you consider the spy/treason tech useful. It could imply that not being religious make you more successful.
The Saracen unique technology in The Conquerors is called Zealotry. And its effect is breedingstronger camels. You know, the less we look into this is probably for the better.