The contents of this post were written for Forts & Engineers 1.1.0 for Victoria 3 1.4 and older.
Updated content for FAE 2.0.0 for Vic3 1.5+ will be made available in the future.

FAE 2.0.0 adds new Siege Ability and Fort Entrenchment stats; both of these are explained in detail in-game through the use of game concepts. Please refer to that for reference for the time being. Most other details should be the same as previous versions.

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The new Fortifications building (hereon interchangeably referred to as "Fort" or "forts") provides the core functionality upon which FAE is built around. Fortifications should not be thought of as a single building, but rather a collection or network of buildings and field works.

It is a military building in a similar vein to Naval Bases in that they can be built at any time, provided the prerequisite technologies are researched, and only up to a certain limit. Once built, they can be customized in a similar fashion to all buildings through the tweaking of production methods. Any given fort's effects in battle can be seen as statistics/modifiers shown in the building output.

Fort Levels
The maximum level a fort can be is 10. Upon initially researching forts, the level cap will be very low. Much like Naval Bases, the maximum level of a fort per-state can be raised through researching technologies.

The following technologies will raise the max level of forts:
• Napoleonic Warfare: +1 (also unlocks forts, capping forts at 1 level when initially researched)
• Field Works: +1
• Breech-Loading Artillery: +2
• Defense in Depth: +3
• Concrete Fortifications: +3

Fort Production Method Groups
Forts come with five unique production method groups, each with their own unique production methods. They are:

• Fort Design
• Construction Material
• Heavy Armament
• Light Armament
• Operational Status

Fort Design represents the physical layout and design of your forts. Its PMs simulate the evolution of fort design from old bastion (star-shaped) forts to bunker & pillbox networks as the firepower of siege weaponry increases over time.

Construction Material represents, as the name suggests, the materials with which the forts are built. By default, they will start off as iron-based constructions and requiring iron for maintenance, but technological research will unlock the ability to trade the iron for steel instead. For low-cost defenses, a wooden option is also available immediately alongside the iron one.

Heavy Armament represents large guns and artillery installed permanently at forts to defend against enemy attacks. Its PMs simulate the evolution from small cannons mounted utop walls to large turret-mounted guns.

Light Armament represents small arms wielded by soldiers stationed within the forts themselves, firing upon enemies as they approach the fort. PMs in this group range from small arms only (i.e. the rifles directly in soldiers' hands) to automatic machine guns.

Each of the four PM groups described above will add to the Defense provided by the fort. They also have the following effects:
• Training Rate: Fort Design & Construction Material - as the PMs become more advanced, the "training rate" reduces to simulate the increasing time demands of constructing and repairing more sophisticated defenses.
• Provinces Lost: Fort Design - more advanced defenses will be more effective at slowing the progress of your enemies.
• Kill Rate: Heavy & Light Armament - stronger weaponry installed in your forts are more effective at killing enemy troops.

Mothballing
There is a fifth production method group: Operational Status. This PMG contains only two PMs: Normal Operations, and Mothballed. Inspired by the mothballing feature from EU4, these PMs control whether a fort is considered to be in active service or in a mothballed state.

Forts running under Normal Operations will, as the name implies, run normally. Their bonuses will be provided to defenders in battle as described in a future section. The forts will be fully staffed and will require all goods necessary to maintain them.

Forts that are mothballed will instead be placed in a deactivated state. They will not contribute to any battles, leaving defenders in the region to fight as if the fort does not even exist. However, the staffing requirement will be significantly lower, and only a fraction of the goods will be needed for minimal maintenance over time.

Forts take a significant amount of time to construct. By mothballing, players can avoid demolishing and reconstructing forts should they need to reduce military expenditures to save money. When the time comes, the forts can be reactivated immediately without needing to wait for reconstruction.

However, recommissioning forts does not come without costs, as explained in the next section.

Redesign & Rearmament
When upgrading to a new PM or upon recommission of a fort from a mothballed state, forts will receive a modifier similar to that of Barracks where the Defense of a fort is significantly reduced for a period of time, recovering slowly for the duration of the modifier. Players must plan their fort upgrades and recommissions carefully, lest they are attacked by an enemy at an inopportune moment.

For recommissions specifically, forts also receive a temporary decaying modifier which reduces the amount of small arms, artillery, and ammunition demanded by the fort to represent the "easing in" of the fort's new staff and reinstallation of armaments.

Combat Engineers are a new specialist company PM added to Barracks and Conscription Centers. Unlocked with the Field Works technology, Combat Engineers (or just "engineers") are a way to mitigate the threat of forts in enemy territory.

On the surface, they add small bonuses to Offense, Defense, and Provinces Captured. These work in a standard fashion to all other Barracks PMs, and thus apply to all battles.

Behind the scenes, they play a very powerful role in directly reducing the bonuses defenders receive from forts in battle. The amount of this reduction is directly related to the ratio of engineers present in battle. Its exact mechanics will be discussed in a future section.

To help the fortunate and/or well-rehearsed commander, a pair of new battle conditions are added, both named "Fort Breached". One exists for the attacker while another exists for the defender. Only the attacker's battle condition may be randomly picked, and the defender's battle condition has a very high likelihood (practically 100%) of occuring when the attackers have their battle condition. That is to say: the attackers must get Fort Breached first, then the defenders will get Fort Breached as well.

For the attackers, Fort Breached adds the following bonuses:
• +10% Kill Rate
• +50% Offense
• +10% Provinces Captured

For the defenders, Fort Breached adds:
• +30% Casualties Taken
• -25% Defense
• -10% Kill Rate

In tandem, these battle conditions are meant to represent the storming of a single fort building, where the defenders become overwhelmed and encircled, leading to significant losses for them and major gains for the attackers.

As stated before, Fort Breached (defenders) appears only while the attackers have Fort Breached (attackers). Fort Breached (attackers) happens randomly in land battles, much like the other battle conditions, but its odds can be improved with the following:
• More than 50% usage of Combat Engineers in battle from Barracks and Conscription Centers combined
• More than 50% usage of Infiltrators in battle from Barracks or Conscription Centers
• More than 50% usage of Mechanized Infantry in battle from Barracks or Conscription Centers if the forts being attacked into are made of wood or iron, with doubled the effect if wooden
• More than 50% usage of Siege Artillery in battle from Barracks or Conscription Centers, with a greater effect if the forts are made of iron, and a greater effect above that if wooden
• Attacking general has the Combat Engineer trait (see below)

It should be noted that Fort Breached can only be rolled if an active fort is involved in the battle. If there is no fort present or if it is mothballed, this battle condition will never occur.

Not to be confused with the engineers specialist company, generals can now obtain a new set of Combat Engineer character traits. Similar to some other general traits, there are three tiers: Combat Engineer, Experienced Combat Engineer, and Expert Combat Engineer, where each tier will grant a bigger bonus.

Each tier will grant a bonus in the following:
• Increased chance of the Fort Breached (attackers) battle condition occuring
• Increased chance of the Dug In battle condition occuring
• Reduced chance of the Mud battle condition occuring

Combat Engineer (i.e. the first tier trait) can be obtained by any character if the following conditions are met:
• The character is a general (this includes country leaders who currently have command of troops)
• Field Works has been researched

The following will increase the chances of Combat Engineer being earned:
• The character has the Engineer trait (a vanilla character trait; curiously, at the time of writing, only Trade Unions agitators have a bonus for rolling this trait)
• Trench Works has been researched
• The character has the Stalwart Defender, Trench Rat, or Defense in Depth Specialist traits, with a greater bonus in the higher tiers of these traits
• More than 50% usage of Combat Engineers PMs in their units from Barracks or Conscription Centers

The list above will also positively affect the odds of a character with Combat Engineer or Experienced Combat Engineer upgrading their trait to the next tier.

For Expert Combat Engineer exclusively, the chances of upgrading to it from Experienced Combat Engineer is also improved if Stormtroopers has been researched.

Whenever a battle starts, the following checks are made:
• Does the state in which the battle is taking place have a fort built in it?
• Is this fort owned by either the country currently defending in this battle or a war ally of the country defending in this battle?
• Is the fort NOT mothballed?

If all conditions listed are met, then there is considered to be a fort active in this battle, and it will provide bonuses to the defenders.

Fort bonuses start by taking the current output of the fort at the moment the battle starts - this means its Defense, Kill Rate, and Provinces Lost bonuses are taken as is from the current PMs, but it also means any goods shortages or modifiers affecting the stats will factor in as well. For technical reasons, temporary changes to the output (e.g. decaying modifiers) will not be reflected in the modifier applied to the defenders for the duration of the battle, even if they continue to change in the fort's information panel at the same time.

A check against the attackers is then made to determine what percentage of their units participating in the battle has Combat Engineers. A linearly-related modifier against the fort bonuses are applied based on the percentage of Combat Engineers present. At 0% engineers (i.e. the attackers have no engineers), 100% of the fort bonuses are granted to the defenders. At 100% engineers, only 25% of the fort bonuses are granted.

Unlike in certain vanilla calculations, the Combat Engineers usage percentage is calculated from both Barracks and Conscription Centers and the two are combined for the final percentage (vanilla likes to only consider those from Barracks). For technical reasons, the percentage is floored (i.e. rounded down) to the nearest 5%, with the exceptional case where 1-4% usage is rounded down to 1% instead of 0%.

The fort bonuses are then applied to the defending general, if present, as a trio of modifiers - one for Defense, Kill Rate, and Provinces Lost, each. These modifiers are removed automatically once the battle ends.

Additionally, the fort involved in the battle also gains a modifier to significantly increase the mortality of all employees working in the forts to simulate battle casualties. This modifier is also removed automatically once the battle ends.