So, Ottomans, the historically large empire who had massive manpower and war focus, in eu4, unstoppable behemoth with invincible armies that takes too much effort to lose, (I have 2k hours in Eu4, so don't reply "oH iF yOu PlAy As AuStrIA Pu BoHemIa AnD hUngArY YoU cAn EasIlY DeFeAt OtToMaNs")
the problem is not with how to stop ottomans, it is historically inaccuracies. So here let me summarise it quickly:
in game, 20k ottoman army is enough to beat 30k, 40k any other army in the game, so here is what's wrong with it,
Let's start with 1444 start date,
Battle of Varna: where many European nations teamed up against Ottomans to stop them, it is 37k against 25k battle, so for this part we can say game is doing OK job.
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so, let's continue with what we can do in 1444, not pre start date, according to EU4, ottomans should NEVER lose against army of smaller size in first few decades of the game. But here are some examples
FALL OF CONSTANTINOPLE
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70k against 7k, with ottomans losing heavy losses, and Constantinople civilians getting enslaved, but according to game, if 70k ottomans go against 7k byzantines,they would just stackwipe and win, while in reality they lose heavily, this means 30k to 50k
So, you might say "this is siege, show me battles, just as eu4 depicts" so here you go:
BATTLE OF VASLUI
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50k against roughly 85k, Moldovans lose 5k while ottomans lose 45k, without counting wounded, this is just impossible in game even if you had 6 6 6 6 general, let's continue with another:
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battle of Targovişte, 30.000 Wallachians against 250.000 Ottomans, (This is ridiclous isn't it?) Ottomans lose 5 times more than wallachians does, can this be done in game? well if you even tried 30k against 30k against ottomans, you would lose 15k while ottomans lose 8k or so, not even close to reality. let's continue:
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Siege of Belgrade, just 1-2 decade prior to game's start date, 4k soldiers against 50k ottomans, with 60k civlians helping Hungarians, and this battle is led by incredible 6 4 6 ruler of Ottomans!!! (I don't even know why he has such incredible stats, as we will see later, he was not that great) you would assume ottomans wiping out entire Hungarian army right? well, then you should stop believing in EU4, because 60k peasants destroy proffesional Sipahis and janisarries.
So, now continue with what I said about Mehmed the conqueror, the god tier ruler in EU4, you will see why he is not great now:
BATTLES OF SKANDERBEG
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8k against 80k, 1 to 10, with %30 of Ottoman troops dying or getting captured, is this possible in game? well you would get stackwiped if you had tried this, so no, let's continue:
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Skanderbeg again, 1 to 4, losing 2k, killing 15k this is not possible again. even with Skanderbeg, so let's continue:
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Now this is epic, 8k against 100k, and god tier Mehmed and Murad leading battle themselves! But getting 20k casualities during siege, and many more during retreat phase, while Albanians getting 1k killed, in eu4, this means stackwipe, but in reality it was albanian victory.
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so now, Mehmed II with his commander leads 2nd siege of Kruje, which they outnumber massively but loses again! even if we take lowest of assumption, 30k for ottomans, it is still impossible for 13k albanians win.
So you might say "but this was skanderbeg, and game made him really good commander, what we can do? you are wrong, because Ottomans tried to take Albania for 2 decades more after Skanderbeg died, let's look at those battles:
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tens of thousands of Ottomans, led by Mehmed, 6 4 6 ruler, again loses to few thousand Albanians, that is not led by Skanderbeg, So, it is not with skanderbeg alone, but with ottoman forces, but let's continue:
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Ottomans at Kruje again, for fourth time! This time Mehmed (that 6 4 6 ruler) Brilliantly uses mass human waves tactics again! But no worries, his usage of mass human waves works this time in 10x outnumbering army! good job Mehmed! I surely couldn't do it with 58k against 2k, you really deserve 6 4 6 stats!
so that was it with balkans, ottoman victories all had massive number advantages apart from Varna, which ottomans had 37k against 30k, still outnumbering thou, so let's continue with Ottoman Venetian wars:
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Battle of Gallipoli! (not 1915 one

10 galley against 32 galley and galleots, but 12 killed against 4000 killed and 1100 enslaved, and most of ottoman fleet is captured! and this is in 1416! 499 years before actual Gallipoli.
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oh my! 55 against 5000? this is clearly ottoman victory! oh wait it is not?? it can't be, eu4 didn't taught me this!
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battle of Focchies, ottomans outnumber as usual, but loses most of their fleet! But this is impossible in eu4 right? it shouldn't be true!
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Battle of Lepanto, the battle that halted Turks, but this is impossible, how 212 ship can destroy most of 278 ottoman fleet? and 9 to 1 outnumbering in galliot number! just impossible in eu4.
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Battle of Candia, in Turkish books, it is just considered as "the battle that we won" but this is reality, and if you wonder why there is 80k turks and 120k turkish casualities, it is because ottoman empire constantly reinforced that army. But nevertheless, Turkish victory
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Siege of Farmagusta, 8.5k against 100k, with 8k killed against 52k killed, oh no, what is this? how can ottomans lose this with massive national idea bonuses they have?!
So that was it with venetian wars, now let's check few Austrian and few Persian wars because I am tired now.
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the brilliant Eugene of savoy, 100k against 210k, and wins the battle! But oh no, Ottomans attack Austria again after spanish war of Succession, and Austria is exhausted of constant wars, what are they going to do???
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battle of Zenta B) Eugene of Savoy decides to ambush invading ottoman troops, and this is turns out to be end of Ottoman expansion to Europe. 429 killed against 30k. Maybe Ottomans shouldn've attacked Austria after seeing they were defeated by french.
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Battle of Petrovaradin, I don't have to say anything, you see it.
So that was with Austria, but let's continue with Persia, specifically Nader Shah's campaigns.
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Campaign of Caucasus.
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Battle of Yeghevardi, where Ottomans attack against numerically inferior nader shah with 50k sipahi and 30k janissary, but Nader shah is encamped in high ground! [shouldn't have even tried it!]
and most proffesional army of the ottomans lay dead.
But for the Persians, I can say that this was not like this all the time due to Persians not having westernised and having little firepower. So Half of the Perso Ottoman wars, persians win and other half ottomans. But this is sign that it was not due to technologically advanced europe, even technologically behind Persia defeated Ottomans with lesser men, and results are clear. Anyways, that was all I can say now, it was exhaustive for me.
oh and lastly, I am from Türkey, you can see it in my profile, so you may ask: "why you do this to your own history?" and my reply is :
you might see those ottoman men are just as "numbers" but every one of them had their own personality, their own life, maybe different destiny, but Ottomans took them forcibly, made them soldier and used them in mass assault tactics that didn't cared for individuals, it was really tough to be soldier in ottoman empire, fortunately Atatürk ended all this warmonger empire and made us more modern and humanist. I am disgusted when seeing Ottoman government saw my ancestors as mere "numbers" in battles. And makes me angry, I am glad all that Ottoman empire ended.
last lastly: "oH nO yOu UsE WiKipEdIa, YoU aRe WrOnG AnD bIaSeD" guys, you can check those battles in archives if you want, pls don't write this 1000000x times over and over, if you are curious if they are real or not, just look at archives and or check sources in wikipedia yourself and point them, and correct them if you want. I used wikipedia because it is easier to show you stuff, and it is clear and neat, I don't want to bother you with extremely hard to read sources. ALSO you can check all battles of Ottomans youself, and you will see that I am not randomly picking battles,%90 battles are like this.
I am historian, not developer, and I hope you developers find a way to fix this issue, it is extremely unrealistic, not fun because ottomans are unstoppable and easy to fix I assume, just boost their manpower and maintainence and lower their quality or idk, you are developers here.