Battle Brothers can be thought of as a turn based simulator of medieval/light fantasy battles. As with many turn based strategy games, it can be split into 2 parts:
1)'turn based' combat or tactical part on a hex tiled battlefield
2)'real time with pause' management or strategy part on the overland map and settlements
As the player progresses through the game, 3 general parameters are improved:
1)XP (earned from battles) which leads to more levels, stats and perks for the men
2)Gold (earned from completing contracts, battles and selling items) which is used to hire men, purchase supplies (tools/medicine/food/etc) and also better weapons/armor/equipment
3)Renown which modifies contract payout and Relations with various settlements which modify buying and selling prices
On higher combat difficulty harder enemy groups appear sooner.
On Beginner combat difficulty player gets 5 points lower (better) "to hit" rolls and enemies get 5 points worse ones.
Beginner combat difficulty and Ironman mode delay the late game crisis by 5 days each.
Increasing Economic difficulty makes contracts pay 10% less per difficulty level. Beginner also increases resource carrying capacity (+200 max ammo, +50 max tools, +50 max medicines) whereas on Expert, deserters take their equipment with them. Selling prices for various items at settlements are also reduced by 10% on Expert.
For starting gold and supplies, difficulty is tiered as high, medium and low (previously beginner/veteran/expert in older builds).
Optionally, you can enable the "Ironman" mode for each difficulty level when starting a new campaign.
With this mode enabled for a campaign, the game saves automatically and the player is not able to set manual savepoints. Therefore the player has to live with every decision he makes and can not reload to a previous state other than the last autosave.
Things that are NOT modified by chosen difficulty levels:
- Enemies will carry the same equipment with the same stats on all difficulty levels
- Availability of items and supplies from towns and cities is unaffected by difficulty
Levels and XP
- See full Article: Level and Experience
The character's level is a value between 1 and 11 and measures experience in battle. Characters rise in levels as they gain experience and are able to increase their attributes and gain perks that make them better at the mercenary profession.
The introduction of "Veteran Levels" allows mercenaries to level even farther than level 11. Every level above 11 (= Veteran Level) requires 4000+1000*(current_lvl-11) XP and there is NO perk point awarded and the stat rolls are reduced to a maximum of +1 for three stats. Veteran levels are capped at L33.
- Attributes can be divided into two different categories:
- Primary Attributes can be increased each level by 1-3 points
- Melee Skill
- Melee Defense
- Secondary Attributes can be increased each level by 2-4 points
- Ranged Skill
- Ranged Defense
- Initiative (3-5 points)
Talent stars further modify these basic ranges.
Some recruits with military backgrounds may already have a few levels under their belt. If you hire a brother who already has one or more levels, you can distribute his perk points and stat roll level ups after you hired him as you do with a normal mercenary.
- Examples of some backgrounds:
- See full article: XP awards for enemies
XP gain in combat: The person who did the killing blow gets 20% of the "XP value of an enemy". The other 80% is distributed equally amongst all men who are still alive.
Details on "total XP value" for various enemies can be found here: XP awards for enemies.
Upkeep and Food Supply
Your soldiers aren't noble adventurers fighting for truth or justice, they are in it for the money. A company without money and supplies will quickly have no members left.
Every soldier gets paid a daily wage of crowns, if you fail to pay them their mood will decrease and they may decide to desert your company.
The wage is increased by 2 crowns per level up and the character trait greedy increases the wage right away by 2 crowns.
Each brother consumes 2 units of food per day unless they have certain traits like spartan or gluttonous.
If you fail to feed them, their mood will decrease and after a while and if angered enough, they may decide to desert your company.
The quality of food currently doesn't have any effect, although there is an event ingame, that when you have a good diversity of food (also with high quality food like venison) that will boost the mood of your men.
High quality provisions usually take longer to spoil than lower quality provisions. Your mercenaries will always eat or drink the provisions which are the closest to spoil.
- See full article: Morale
Battle Brothers uses a day/night cycle, going through the phases
- Dawn - Morning - Midday - Afternoon - Evening - Dusk - Night
Settlement vendors (e.g. Temples, Training Halls, Barbers, etc) are closed and not available during the Night portion of the day/night cycle. It is not possible to recruit new mercenaries during the Night either. Taverns are the only settlement building open at any hour of the in-game day/night cycle.
A complete day in-game is about 1 minute 45 seconds in real time. Use the Space Bar or click the pause button next to the clock at the top of the screen to pause the game. The passage of time is also halted while in town, while in an event dialogue, or during combat. While the game is not paused on the world map, provisions will be consumed, damaged gear flagged for repair will be repaired (provided you have Tools and Supplies) and wounds will heal (provided you have Medical Supplies). Wages are paid to your mercenaries daily at noon (once Afternoon starts).
Travel on the overland map is slowed during Night and sight radius is reduced, making unexpected attacks by enemy groups more likely.
Enemy Troop Size and Composition
Enemy troop sizes are influenced by many factors, the overarching one being Combat difficulty which is set at the start of the game, which increases both numbers and tier (more higher tier mobs like Brigand Leaders).
Difficulty/enemy troop size of contracts, non-contract locations and roaming parties scale differently.
Contract difficulty scales with the strength of your roster (which equals a sum of individual strengths [10 + 2*(lvl-1)] of 12 strongest brothers), the difficulty rating (skulls), the type of contract, and some factors specific to the type of contract – for example, escort contracts are generally more difficult the longer you travel, as it increases the chance to run into enemies. Renown does not factor into contract difficulty, but it does factor heavily into contract rewards; the more renown you have, the more you'll be paid. Contract reward also scales with the difficulty rating (skulls) of contracts, which is why it offers some indication as to the contract difficulty.
Difficulty of non-contract locations and roaming parties scales mostly with distance to civilization and time. Roaming parties also scale slightly with player strength, but less so than contracts, and non-contract locations don't scale with player strength at all.
Therefore, contract-spawned mobs and locations are always closely matched to the player's party and offer an appropriate challenge. As non-contract locations do not scale to the player's party strength, far-off locations will probably be risky or difficult early in the game, whereas in the late game, these become juicier targets for unique hunting, especially if the player has levelled up speedily.
Roster strength or party strength as mentioned above, takes into account number of men and their levels. 
Based on in-game tests, roster strength has a very significant impact on enemy troop size and composition. 
From further testing, gear does not appear to have an impact on roster strength.
Mousing over lairs usually reveals rough assessments of numbers (detailed in table below) and troop types for the defenders.
See full Article: Factions & Relations
Your Renown is your repute as a professional mercenary company and reflects how reliable and competent people judge you to be. The higher your Renown, the higher the reward from Contracts and the more difficult Contracts people will entrust you with, and the more resources your enemies will spend on working against you. Renown increases on successfully completing Contracts and winning battles, and decreases on failing to do so.
|Daily Renown Decay||-3|
|Retreat from Combat||-15|
|Contract Poorly Completed||-25|
|Failing a Contract||-75|
|Canceling a Contract||-100|
|Betrayal on Contract||-200|
|Winning a Fight||+5|
|Destroying a Location||+10|
Market prices and economics
While Battle Brothers is primarily a turn based tactical simulator, managing your funds well will go a long way towards having an easier time in hiring good men and gearing up. Buy/Sell prices of items in markets are largely determined by:
The general scheme is to buy cheaply/restock supplies at smaller settlements (trade goods, tools/medicine/etc) and and sell loot/trade goods at a profit at larger settlements. Repairing items before selling is favorable if the increase in selling price exceeds cost of tools used in repairs. Building good Relations with these settlements by doing contracts for them and taking advantage of favorable settlement situation modifiers will further increase profit margin.
A more detailed guide can be found here.