Some apparent random strengths and weaknesses of civilizations and units match well with History. This game is an amazing example of Shown Their Work:
Building castles is a massive returning investment in the Castle Age (roughly 11th-14th century), when they are almost invulnerable. By the late game, they become easy prey of trebuchets and cannons (after researching chemistry), mirroring their decline in the late 15th century after the popularization of gunpowder.
War Elephants are irritantingly vulnerable to conversion by monks. This seems stupid (especially since the elephants have no rider) until you realize that the main weakness of war elephants in real life was that they panicked easily in battle and trampled the soldiers on their own side. Furthermore, enemy commanders went out of their way to capture and bring back at least one elephant alive for the victory parade home - and so do a lot of non-Persian AoK players. Alternatively, the elephant does have a rider (although their sprites don't show them), who is converted to the monks' side.
Mongol Mangudai are weakest against Saracen Mamluks. The Egyptian Mamluks' destruction of the invading Mongol army at Ain Jalut (1260) is usually marked as the end of the period of Mongol expansion.
The Spanish are weak in the early ages, then turn increasingly strong after they hit the Castle Age. This is a mirror of the weakness and irrelevance of the Iberian Christian kingdoms before the 11th century. By the late game, the Spanish turn into one of the strongest civilizations in the game, mirroring Spain's emergence as a world power around 1500. Moreover, the biggest weakness in early Spanish gameplay is the lack of crossbowmen upgrades. While the Medieval Spanish did use crossbows in real life, archery wasn't cultivated there historically as in other countries, and bows often took a backseat to slings and javelins in the battlefield.
The Iberian exception were the Portuguese, who imported English longbowmen (for example, at Aljubarrota) and had an ordinance forcing all towns to provide the King with a number of trained, professional crossbowmen should he need them, the Besteiros do Conto. While the game's Portuguese don't have access to longbowmen, they do have a full foot archer roster that includes Crossbowmen and Arbalests, unlike the Spanish.
On land, the Vikings are stronger in the earlier ages and become weak in Imperial. At sea, they are among the most powerfull through the game. In other words, as the game goes on, Vikings are better at landing, raiding and leaving in their boats when real opposition shows up. Their UU is not good at sustaining a long fight, but its self-healing makes it good to annoy other players with a raid after another, not needing to place them in buildings or to carry monks along to heal the injured.
The Italians counter the Turks strongly, due to their reliance on light cavalry and gunpowder units like bombard cannons and janissaries. The Italians' Genoese Crossbowman counters the Turks beefy Horse Archers and melee cavalry, which is made more powerful because the Turks don't have access to Elite Skirmishers or Onager upgrades. Their other UU, the Condottiero, is a special infantry with bonus attack against gunpowder units. True to history, Italian mercenaries played an important role in slowing the expansion of the Ottoman empire in Europe and were formidable foes to the Ottoman army, as evidenced in Lepanto.
The Celts being specialized in destroying castles is probably taken from Robert the Bruce's rebellion, which consisted mostly of the Scots seizing their own castles from the English and burning them down.
The Spanish wonder is the Tower of Gold. At first glance, this seems a cheap reference to Conquistadors and an oversight - the building is famously of Muslim (Almohad) origin, while the game Spanish stand in for the Iberian Christian kingdoms. But in truth, only the lower section of the tower is Almohad: the middle one was added by the Castilians in the 14th century and the upper one after the conquest of the Americas. Thus the Spanish wonder is actually a clever summation of Spain's history in the game's time period.
The Conquistador also starts making more sense if you pretend that it isn't really a mounted gunner but a mounted javelineer... like the "Genitour" it replaced during development. Players have traditionally considered it disappointing because it is too weak and has too little range to win battles by itself and acts best as an auxiliary to heavy cavalry, raiding villagers, or harassing infantry with hit and run tactics, but without the finesse and security of a cavalry archer. In other words, exactly what the historical light cavalry, javelin-throwing Spanish "Jinetes" were. The decision to use guns instead of javelins was perhaps motivated by consistency, because the vanilla had used hand cannoneers as infantry counters and foot javelineers (Skirmishers) as archer counters (instead of using Slingers like in Rise of Rome), but a ranged anti-archer unit would have been indeed too weak to merit being the main UU of a civilization. A good alternate solution would have been to still give the Spanish Jinetes, but with the stats and abilities of the Burmese Arambai introduced in Rise of the Rajas.
The lack of Heresy (a Monastery technology that kills your units instantly instead of being converted by enemy monks) in the Italians may seem far-fetched since Italy is well-known as the home of the Roman Catholic church that in the Medieval period had strong political influence throughout Italy and most of Europe. However, Italy is also the home of The Renaissance, in which scholars challenged the authority of the Catholic Church.
Not to mention Italian armies were made mostly of mercenaries in real life, and those can't be expected to be so loyal as to choose death once captured, rather than just getting their paycheck elsewhere.
Believe it or not, as far as the game's engine is concerned camel riders are ships, and they are far more vulnerable to towers and castles than horsemen as a result. Is this supposed to account for camels' historically limited role in warfare, used mostly to transport supplies and harassing travelling enemy troops out in the desert, far from urban centers? A pun on camels being sometimes called "the ships of the desert"? Both?
After African Kingdoms, camels get their own armor type.
Apparently, it was planned once that camels would have the ability to captureenemy villagers, but this was scrapped during development. The vulnerability to building arrows may be a harbinger from that, as a way to limit camels raiding to areas far from towns and fortifications.
The Portuguese are actually well thought out as a faction. They were the first European power to begin exploration and colonization, especially along the coasts of Africa, so the feitoria building really makes sense, because no matter what, there are always resources flowing in from *somewhere* (the colonies). And if you rely too much on Feitorias, they will fill out a lot of your population slots. Also, historically, the Portuguese were almost always outnumbered in any fights and wars they got themselves into, not to mention having a relatively low population when in comparison to their European peers. However, the Portuguese unique units are good at crowd control, so the feitorias giving you a bunch of resources from off map, while reducing your population, and having units that make up for possibly being outnumbered, really fits with the general history of the country.
Ever wondered how is it possible that if you kill an animal with a villager you can harvest it for food while killing it with a military unit will automatically turn it into a shallow carcass? It may be because any military unit may want to pick the head of the killed animal in order to display it as a trophy... or it may be because the villagers are skilled hunters, so they know how to kill an animal without damaging it for food recovery, whereas a military unit would just hack it in order to make it edibleless.
A soldier on campaign may also keep the meat for himself and not be compelled to hand it over to his lord, like a peasant would.
Soldiers may also be doing it For the Lulz and intentionally leaving the meat to rot, rather than hassle themselves bringing it back.
The Vietnamese are classified as a Stone Wall civilization, yet they do not have access two important building upgrades (Masonry and Architecture) that makes their buildings more durable. This is often reflected on the fact throughout Vietnamese history, whenever a major power or empire like China, France, or Japan have occupied and invaded Vietnam, many of Vietnam's major cities are often easily occupied by the invaders. Instead, much of Vietnamese's defensive capacity comes from their heavy use of Hit-and-Run Tactics and their quickly deployed army that often consist of peasants and volunteers (which is reflected with their beefy archers, their Imperial Skirmisher upgrade, and free Conscription civilization bonuses) and many of their base of operations happen to be located in secluded and isolated areas. This is also highlighted in many of the scenarios in the Lei Loi campaign in the game, where most of your starting positions happen in small, isolated areas, while the Ming and your opponent starting positions happen to be fortified towns and cities that have been occupied by the Ming dynasty.
Likewise, the Vietnamese as a civilization are considered to be a Magikarp Power civilization due to the fact the Vietnamese faced various invasions from powerful empires such as the Mongols and were occupied by the Ming dynasty, but the Vietnamese became formidable and strong when they drove out the Ming occupiers.