Game Breaker / Crusader Kings

• North Korea mode, a popular strategy which circumvents a lot of the restrictions on ruler actions by adopting a reign of tyranny, stripping lands from vassals, forcibly passing laws they'd never agree to, and executing or banishing anyone who objects. In the short term, it's easy to replace the now-disloyal followers with more loyal ones, or at the very least, find a skilled spymaster and chancellor, find ways to make them loyal to you, and have them counter plots; your angry nobles will be plotting to kill you right off the bat, but as long as your spymaster is sufficiently skilled he can stop these plots dead in their tracks, and if you can identify the conspirators you can simply order them dead. Alternatively, wait and do this at the tail end of your ruler's lifespan; when he dies, none of the negative penalties are passed on to your heir (until the 2.5 patch, which added a temporary buff or debuff for vassal opinion giving the heir half the opinion the vassal had of his predecessor, though elective succession is exempt), and you now have a kingdom with vastly more favorable laws, all of the territory under your control, most of your hated enemies dead or banished, and your new ruler suffers none of the penalties associated with any of it. Some players even take this a step further and go tyrant their entire campaign, resulting in a brutally rich and powerful dynasty with massive armies, loads of taxes, and a long line of corpses behind them.
  • Note that an autocratic and hegemonial rule as described here is only a Rule of tyranny. North Korea mode refers to an Awesome, but Impractical ruling style that involves holding all counties in one's own territory by oneself, i.e. not having any vassals whatsoever who have the ability to rebel (and thus, no rebellions, factions, etc.) However, depending on demesne size the penalties incurred from doing so might be so large that it can completely negate the monthly income and prestige bonuses one gets from their holdings. A not quite as popular strategy called Guantanamo Mode is to keep all vassals who can rebel imprisoned at all time, the drawback here being that every attempt at imprisoning somebody will cost you 10 piety, and that failed imprisonment typically leads to rebellion.
    • And now that the game allows you to set the demesne size to unlimited at the start of the game, it's no longer impractical.
• Muslims in general have a lot of advantages in early starts.
  • Your basic Muslim ruler has Iqta government instead of normal Feudal, which allows nobles to freely hold mosques instead of only castles, meaning higher income from both the temple(s) and the fact that Iqta tax law defaults to 10% on both castles and mosques, making them usually richer than any Christian nation. ^note  While the Conclave DLC changed it such that Christian nobles will pay some tax by default, Catholic clergy still retain their unique mechanic of only paying taxes to the Pope and withholding their levies, if they like the Pope more than their secular liege. Needless to say, a pious Pope would be a significant drain on both income and levies for Catholic realms. Muslims are restricted to open succession unless the council is fully empowered, meaning that the vassals cannot form factions to change the succession law (a source of civil wars) ^note In addition, at early start dates, many Christian realms use gavelkind succession, which can easily split apart large realms when succession takes place. In early start dates, they also begin with two strong realms, the Umayyads in the west and the Abbasids in the east, whereas Catholic Europe is very divided and it's common to see Charlemagne die before he can form the Holy Roman Empire to rival the Umayyads, and only the Byzantine Empire poses a significant threat to the Abbasids. In addition, the game doesn't model the things that historically made the Spanish Visigoths not worth the Moors' trouble to conquer (i.e. that they were poor and good guerrilla fighters). ^note If anything, game mechanics make Asturias an even more attractive goal to conquer. First of all, Asturias lies in the same de jure empire (Hispania) as the territory the Umayyads control, which meant that the Muslims could muster more levies from the territory once it is conquered, as compared to other regions which lie outside Hispania. Next, a special creation condition for the Empire of Hispania is that its constituent kingdoms are not held by infidels. Thus, for the Umayyads to form Hispania, they have to fight the Spanish Visigoths until they at least lose the Asturian crown.
  • Muslim merchant republics are exempt from the Decadence mechanic and patricians can designate heirs or rely on seniority, while polygamy means plenty of sons for extra trade posts (potentially magnified even further with the Seduction focus), and Islam's religious casus belli mean one need not rely on the underpowered CBs of merchant republics for expansion. For bonus points, play as Shi'a or Ibadi to get holy war CBs on most of the Muslim world in addition to infidels.
  • The Yazidi heresy of Sunni Islam. If you can manage to become the Yazidi sheikh, you get holy war casus belli against basically the entire world, the ability to call jihads, plus the ability to excommunicate fellow Yazidis you don't like. (Of course, like vassalizing the Pope, getting there is extremely difficult: only two counties in the game are ever majority Yazidi by default.)
  • With Monks & Mystics, rising through the ranks of the Assassins (which requires either being a Shi'ite or becoming a Secret Shi'ite as part of the recruitment event) makes having people murdered progressively easier, to the point where at the top rank it becomes ridiculously easy to gain 300% plot power or more against literally anybody in diplomatic range.
• A relatively infamous one is the Horse Archer unit, a combat unit type mostly exclusive to Altaic and Turkic cultures. It is by far the most powerful unit in the Skirmish phase of combat in the game (which is the phase every single battle starts in) and does an obnoxious amount of damage. All elite knights in the world are of no use if they get massacred before they can even reach their enemy. Even worse, Altaic cultures also have what is generally considered to be the most dangerous culture-specific combat tactic in the game, Retreat and Ambush, which besides boosting the Horse Archers' damage output by 200% also immediately changes combat to the skirmish phase again. One reason why the Mongols are so bad is because their Doomstacks consist mostly of Horse Archers who are all led by excellent tacticians and will use that tactic to eradicate any resistance on the way.
  • Horse Archers (and regular Archers) got nerfed around the time of Conclave (mainly because they made nomads too much a Game-Breaker themselves), to the degree that they're considered the worst unit in the game from Conclave on. Mainly this is because Horse Archers have atrocious tactical synergy and adding them makes your army worse at the Skirmish phase, and flanks consisting of pure Horse Archers will only use the 'Disorganized Swarm' tactic which is also completely awful. In its place, plain old Light Cavalry became kings of the battlefield, to the degree that pure Light Cavalry (and especially Camels, who are basically slightly better Light Cavalry) will quickly sweep entire battles by continually getting the 'Raid' tactic (which triples their damage output) and cause the enemy to flee long before they hit Melee. Generally speaking, any faction that doesn't have Light Cavalry or Light Infantry (which can use the 'Feint' tactic to deal decent if not overwhelming Skirmish phase damage) retinues is best off building armies based entirely on the Defense retinue, as it is nigh impossible for 'regular' levy troops to significantly damage it.
  • Italian and Scottish pike retinues also score very high as retinues go. See the above section on Defense retinues? Well, Italian or Scottish pikes are basically those, but with some decent punching power on top of it. Because they count as Heavy Infantry, they're also very good at Storming the Castle (unlike Cavalry and Light Infantry retinues who dominate battlefields but are terrible at sieges).
• Nomads, even after nerfs to their most commonly used unit (see above) and to their population and manpower. With Holy Fury, nomads can even force a change in their succession type, by reforming a pagan faith and selecting the relevant doctrines, to the powerful Open succession used by the Muslims. While Open succession is powerful by itself, it is even more significant for nomads as their Nomadic succession disqualifies dynasts of different cultures and children from the title of Khagan (leader of all clans in the realm).
• As a Catholic, having a vassal Pope (which can be achieved by making an Antipope and pushing his claim on Rome, making him the official Pope). Putting aside that you need to be an Emperor to have one in the first place, once you have one, you get free claims on any Catholic duchy or county in the game and the power to excommunicate any Catholic character who annoys you, letting you imprison them penalty-free and making it nigh impossible for them to form factions. This doesn't so much break normal mechanics for vassal management and expansion as snap them neatly in half. Despite being patched in Holy Fury, the patch merely toned down the active use of papal mechanics; Catholic emperors still benefit from vassalization by the fact that since they are the Pope's liege, they cannot be excommunicated. Next, by not asking the Pope for the Game Breaking favours listed above, the Pope is likely to have positive opinion of his liege Emperor; this meant that the Pope will pay taxes to the liege, thus closing the revenue loss due to bishops from the realm paying taxes to the Pope. Even a vassal Pope's tendency to ask for independence in exchange for officiating a coronation ceremony can be mitigated by having Free Investiture, because this law allows the emperor to pick other clergy vassals to officiate, and as a bargaining chip (the vassal Pope might instead ask to instate Papal Investiture).
• As trivial as it sounds, cities located in coastal provinces can build trade harbours. This doubles the wealth generated by said cities, as they have two, instead of one line of economic buildings. Thus by default every coastal province you have is equal with having two inland. And you can always build additional cities, each twice as good as inland ones. Do the math.
• Tribal pagans can spend prestige to make retinue seeds. Unlike regular retinues, these retinues cost prestige maintenance instead of gold, which itself is fairly powerful. Add on to that the fact that gold and prestige are very easily made by raiding, then you have the potential to mass permanent armies numbering in the tens or hundreds of thousands.