Siege

The main siege interface

Sieges are the main way for an army to capture hostile forts and take control over a territory, and usually involves stationing the army on the fort for an extended period of time while waiting for the defenders to run out of supplies and surrender. A siege usually takes a considerable amount of time but can be done with minimal losses as long as the army is properly supplied, contrasting with assaults which are much faster to execute but can result in significantly more losses.

Unfortified territories use the same siege mechanics when being occupied as with fortified territories, though the process is much faster. However, they will tend to revert to the control of whichever country controls the local fort or province capital.

Siege leader

In every siege, there is a single army that is in overall control of the siege. By default, the country that owns the army that started the siege is the siege leader. If a country has an army involved in the siege and is either the warleader (with the siege leader being a secondary participant on its side) or the suzerain of the siege leader, then it can assume control over the siege using an interaction in the siege interface. As well, if the army moves away they will give up control over the siege and pass it over to the next army involved in the siege, which may belong to a different country (or even one that is not involved in the same war).

During a siege, it is the siege leader's army, commander, and country modifiers that are applied, so if there are multiple armies involved it may be advantageous to ensure that the army with the best general is leading the siege. In addition, if the siege is successful it will be the siege leader that takes control over the territory (if they are at war with the owner; if not, then the territory will revert back to the control of its owner), which can be important if two countries are simultaneously at war with the same country and both hoping to take over the same provinces.

Siege phases

A siege progresses in phases, whose length is determined by a number of modifiers. The outcome of each phase is decided semi-randomly at the end based on a dice roll and a number of modifiers, and depending on the result might give increased siege accumulation, losses for either the garrison or besieging army, a breach, or the surrender of the garrison and the end of the siege, or nothing at all. Assuming that the siege has not ended or been paused, a new phase will begin immediately after the current one has been resolved.

A siege phase will only progress as long as there are at least 2000 hostile soldiers (regardless of unit type) in the territory for each fort level. Note that only armies stopped in the territory count; any soldiers that are in the territory but are currently moving are not considered to be part of the siege. If there are not enough soldiers to properly siege the fort, any gained progress will not be lost as long as there are still attacking soldiers but the siege phase timer will be frozen and the current phase will not end until the besieging army becomes large enough again.

Siege interval

A siege phase has a base time of 25 days for fortified territories and 15 days for unfortified territories (note that unfortified territories will always be occupied after the end of the first siege phase). The siege phase time is increased by the defender country's Fort Defense modifier and the local province's and territory's Local Fort Defense modifier, and decreased by the attacking country's and army's Siege Ability modifier. Both modifiers are applied additively and at the same time, which for instance means that a +10% Fort Defense modifier from the defender and a +10% Siege Ability modifier from the attacker will cancel each other out.

Siege progress

At the end of each siege phase, the progress gained in the siege is based on a 14-sided dice roll combined with several modifiers (as with combat). Generally, a siege ends when the dice roll and its modifiers for a particular siege phase adds up to 20; since this is more than the highest possible dice roll, modifiers need to be accumulated to actually complete a siege.

All roll modifiers (including the dice itself) are listed below:

• Dice: 1 - 14
• Siege Accumulation: Up to +11, increases over time based on past dice rolls
• Commander: +1 for every 5 martial skill (rounded down)
• Blockade: -1 for unblockaded ports.
• Fort level: -1 for every fort level.
• Siege Engineering: +1 for each point of the army's and country's Siege Engineers modifier (Up to a maximum possible +8)
• Engineers: +1/(fort level + 1) for each engineer cohort, rounded down
• Breaches: +1 for each breach

Below is a list of outcomes with each dice roll:

• Disease Outbreak (dice 1 and roll < 3, higher priority): No progress, 5% of attackers die.
• Status Quo (roll < 5): No progress.
• Supplies Shortage (roll 5 - 11): +1 to Siege Accumulation, 1% of Garrison dies.
• Food Shortage (roll 12 - 13): +2 to Siege Accumulation, 3% of Garrison dies.
• Water Shortage (roll 14 - 15): +3 to Siege Accumulation, 5% of Garrison dies.
• Defenders desert (roll 16 - 19): +2 to Siege Accumulation, 10% of Garrison dies.
• Breach (dice 14, all modifiers ignored, higher priority): +2 to Siege Accumulation, +1 to Breach, 5% of Garrison dies.
• Surrender (roll 20+ or less than 100 garrison remaining, top priority): Siege over.

Time to capture

Monte Carlo method can be used to simulate the siege system and to estimate how many intervals are required for a capture. 1000000 simulations were ran for each scenario. Capture by garrison running out was extremely unlikely (less than 0.01%).

• Initial: Roll modifier at start of the siege (without siege accumulation).
• Average: How many intervals are needed on average
• Lucky (5th pct): 5% chance to capture the fort with this amount of rolls or less
• Good (25th pct): 25% chance to capture the fort with this amount of rolls or less
• Expected (50th pct): 50% chance to capture the fort with this amount of rolls or less
• Bad (75th pct): 25% chance to capture the fort with this amount of rolls or more
• Unlucky (95th pct): 5% chance to capture the fort with this amount of rolls or more
• Disease: Average amount of diseases per siege.
• Assault: Chance that the siege could be ended with an assault (garrison falls below 50% or 75% with a breach).
• Intervals saved: Average amount of intervals saved when assaulting with above conditions.
• Intervals saved (total): Same as above but also includes sieges where assaults were not possible.

Initial	Average	Lucky	Good	Expected	Bad	Unlucky	Disease	Assault	Intervals saved	Intervals saved (total)
-5	28	13	18	24	34	53	75 %	71 %	13	9.3
-4	19	11	14	17	22	33	57 %	51 %	7.0	3.5
-3	15	9	12	14	17	24	43 %	37 %	4.7	1.7
-2	12	8	10	12	14	19	30 %	27 %	3.5	0.95
-1	10	6	8	10	12	16	19 %	20 %	2.8	0.56
0	8.6	5	7	8	10	13	10 %	14 %	2.4	0.35
1	7.4	4	6	7	9	11	0 %	10 %	2.1	0.21
2	6.4	4	5	6	7	10	0 %	6.7 %	1.9	0.12
3	5.6	3	4	5	7	8	0 %	4.2 %	1.7	0.07
4	4.8	3	4	5	6	7	0 %	2.4 %	1.5	0.04
5	4.2	2	3	4	5	7	0 %	1.3 %	1.4	0.02
6	3.6	1	2	3	5	6	0 %	~0 %	1.2	~0
7	3.1	1	2	3	4	5	0 %	~0 %	1.1	~0
8	2.7	1	2	3	4	5	0 %	~0 %	1.0	~0
9	2.3	1	1	2	3	4	0 %	0 %	1.0	0
10	2.1	1	1	2	3	4	0 %	0 %	1.0	0

Above values multiplied by the length of a siege interval results in the total time to capture.

For example a level 1 fort will be captured in ~8 months with the default 25 day siege interval.

Siege effects

While engaged in siege, all besieging armies get the following modifier:

• +0.10% Monthly Experience Gain
• -3% Experience Decay
• +1% Attrition

In addition, if an army currently besieging a fort is engaged in battle, it will be considered the attacker for the purposes of determining terrain penalties even if it was technically the defender. For instance, an army besieging a fort in a mountain terrain territory will take a significant combat penalty if it is attacked.

While under siege, a territory will also get the following modifiers:

• -30 Migration Attraction
• +10% Pop Demotion Speed
• -150% Local Monthly Food Modifier

Enslavement

After winning a siege or occupying an enemy territory with an army, the attacking army will enslave part of the territory's population, demoting the pops and moving them to the attacker's territories (though they will keep their culture and religion). Enslavement requires the sieging army to have a commander, and the number of pops enslaved depends on the army's Enslavement Efficiency modifier, with a base of +5% that can be increased by the commander's martial skill and traits, adopted military traditions, heritages, wonder effects, and more. Capturing enemy pops (and preventing one's own pops from being enslaved) is often one of the central aims of waging war, and is a powerful way for a country to increase its economic power at the expense of its enemies' even without taking any territory.

The number of each pop type that is enslaved is summarized below:

${\text{Nobles Captured}}={\text{round}}\left({\frac {{\text{Enslavement Efficiency}}*{\text{Total Nobles}}}{5}}\right)$
${\text{Citizens Captured}}={\text{round}}\left({\frac {{\text{Enslavement Efficiency}}*{\text{Total Citizens}}}{5}}\right)$
${\text{Freemen Captured}}={\text{round}}\left({\frac {{\text{Enslavement Efficiency}}*{\text{Total Freemen}}}{5}}\right)$
${\text{Tribesmen Captured}}={\text{round}}\left({\frac {{\text{Enslavement Efficiency}}*{\text{Total Tribesmen}}}{5}}\right)$
${\text{Slaves Captured}}={\text{roundup}}({\text{Enslavement Efficiency}}*{\text{Total Population}})-{\text{Number of Other Pops Captured}}$

In general, the vast majority of pops that are captured will be slave pops, with the other 4 pop types only being enslaved at approximately only 1/5 the rate that slaves are. Note that since an army can only capture as many slave pops as there actually are in the territory, if a territory has disproportionately few slave pops it is possible for an army to capture less pops than you would expect from multiplying the territory's population by the army's enslavement efficiency (though an army will never capture more than expected).

After being enslaved, each captured pop has a 50% chance to get demoted and a 33% chance to die instead of moving to the attacker's territories.

Once part of a territory's population has been enslaved, the territory will be immune to being enslaved again for the next 5 years, which prevents a territory from being completely depopulated if it is sieged and re-sieged several times in quick succession. However, territories that are continually sieged and occupied over a longer period of time still face the risk of demographic devastation and may even become decolonized.

Captured pops will generally tend to be sent to the country's capital, and to a lesser extent territories in the capital and nearby provinces, especially nearby provincial capitals, and capturing pops is in general one of the most effective ways to grow the capital city and region. They will avoid territories that are at or exceeding their population capacity if it all possible.

Mechanics