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I am playing the EU4 demo and I am confused about sieges. I saw a youtube video where the player said that 2000 infantry is enough for a siege. I have several questions about this:

  1. If 2000 infantry are sieging a province, will they win eventually no matter what? (unless of course they are defeated by another army)

  2. If I have more troops, will they win the siege faster?

  3. The attackers and defenders both have a morale bar in the siege window. Is the win condition getting the enemy's morale to zero?

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2000 is enough for any province without fortification. It will take around 1 month game time. There is attrition, meaning that your force may lose men. Since 1000 is the minimum needed, 1000 men could easily become 999, so 2000 avoids this potential problem.

If there is fortification, things are completely different. First of all, 2000 is never enough. How many men you need depends on the fortification level and ranges from 3000 to around 20k. (Again, bring a few more because of attrition). Adding more troops beyond the minimum is useless and usually bad because of attrition (cannons are an exception).

In addition, fortified provinces are not taken over within 1 month usually. You need to win a dice roll, of which you have around 1 per month. The outcome of the dice depends on many factors. You can see the chance to win the siege as a percentage. Some of them are:

  • the level of the fortification (a small fort has a small malus while a large star fort has a very high malus)
  • the number of cannons in your army (more is better, but there is a limit)
  • the siege ability of the leader of your army, if present (more is better)
  • access to the sea, modified by blockade of this access
  • a few other things
  • a timer, that increases over time and after a long time gives an extremely high bonus

The time increases faster the higher the sum of bonuses minus maluses is, you should always try to provide full force instead of simply waiting it out with minimum forces.

For your planning, if the percentage is x%, you will probably win the siege within 100/x turns. For instance, with 20% chance it will probably take 100/20 - 5 turns. 1 turn is around 1 month, the exact span depends on many things.

If you need to reduce your available manpower very quickly(*), you can also assault the castle under certain conditions. For that you need a lot of men. You will lose a ridiculous number of men (something like losing 17k men against a defending garrison of 1200 is not unusual). If either the morale bar or the number of (only foot, not horse) men of one faction become zero, the battle is over.

(*) That was a joke. Do not do this.

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    Your "100/x turns" approximation doesn't take into account rolls that add to the siege progress without winning it. It is also completely invalid for -% rolls, it would suggest negative time – Caleth Jun 18 '18 at 12:34
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    @Caleth That's right, and like tons of other details was left out, as this already became way longer than I wanted it to be for a 'simple' question. I was just trying to give some example and understanding to the number. – mafu Jun 18 '18 at 17:08
  • It might be worth noting that assaulting a fort is actually viable when you're retaking back a fort that was taken from you a few months earlier: burst the walls with 50 mil points and assault the fort then and there to avoid wasting months of time retaking it. It'll then cost you only a few thousand men. – Denis de Bernardy Dec 30 '19 at 19:39
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To expand on mafus excellent answer with some more numbers:

  • The dice you roll is between 1 and 14, whereas rolling a 1 results in a disease outbreak in the besiegers camp, killing 5% of the troops.

  • the dice result is roll + leader modifier + artillery modifier + blockade status - fort level + wall breach + obsolete fort level + garrison bonus + siege status modifier

  • the dice result means the following:

    • under 4 - nothing happens
    • 5-11 - +1 siege status and -1% garrison
    • 12-13 - +2 siege status, -3% garrison
    • 14-15 - +3 siege status, -5% garrison
    • 16-19 - +2 siege status, -10% garrison
    • 20+ - surrender, attacker wins
    • The percentage nubmer shown on the interface is the chance of the final dice result being 20+
  • How the modifiers are determined

    • Leader - 0-6, depending on their siege stat
    • The artillery bonus adds between 1 and 5 (or up to 8 with napoleonic warfare). The exact number depends on the fort level and the number of cannons you bring. however a single cannon regiment will always add +1
    • Blockade - if a province is coastal, blockade is a -2 modifier, if the province isn't blockaded by ships. Otherwise it's a +2. So, that's a swing of four depending on sea control.
    • Fort level - Between -2 (a castle) and -8 (a fortress). The capital province always a level 1 fort, though.
    • Obsolete fort bonus - If you can build better forts than you are sieging, you get this bonus, +1 per level difference. E.g., when you can build a fortress and siege a bastion you get a +2
    • Garrison bonus - If the forts garrison is at less than half strength, this is a +1 bonus
    • Walls breached - adds +2. A breach occurs when roll + artillery bonus/3 + max fort level/10 > 14. Alternatively you can spend 50 military points to force a breach with an artillery barrage.
  • The turn timer maru described as "about a month" depends on the attackers siege ability and the defenders fort defense as well as the tactics difference. The time is 30 days + fort defense percent - seige ability percent +/- 6.25% per 0.25 tactics difference.

  • The siege only progresses when the attacker outnumbers the garrison at least 3:1. Note, that the attacker constantly suffers attrition. If the attackers numbers fall below that threshhold, siege progress immediately stops.

  • Assaulting is, as maru says, generally a bad idea, but do consider it vs very lightly defended forts (less than 500 men garrison).

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