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Due to numerous complaints about how experience complicates battle analysis, leads to errors in assessing troop strength, etc., I will write a brief manual on troop experience.
So, when playing with experience, troops gain levels during battles, from level 0 (no experience) to level 10 (elite).
Newly purchased troops have zero experience. Experience is retained when upgrading. When merging two stacks of the same type into one, the experience of the resulting stack is calculated based on the principle of preserving the total amount of experience. For example, if one stack has 20 units with 10,000 experience, and the other has 60 units with 0 experience, the result will be 80 units with 2,500 experience.
In battles, all troops in the player's kingdom gain experience – not only those in the battle, but also those roaming around or stationed in the castle (recruited). This is what makes experience valuable – it encourages players to engage in as many battles as possible, fight on multiple fronts, actively use roaming heroes, and generally play more aggressively than when playing without experience, where such actions often only increase losses and time spent without a significant benefit.
Experience is only awarded for won battles – experience is not gained when retreating or paying off the enemy. Experience is also not awarded if the battle is won by the castle/mine garrison in the absence of a hero.
Troops gain experience equal to 50% of the experience gained by the hero. The Learning skill only affects the hero's experience and does not affect the troops' experience – they will gain the same amount as without Learning.
When killing upgraded enemy troops, experience is awarded based on their nominal health, without taking into account upgrades or the 25% bonus of special units.
Each troop level has its own experience scale required to reach that level. Lower-level troops level up faster, while higher-level troops level up slower. For example, level 1 troops gain their first promotion at 1000 experience, while level 7 troops gain it at 4000 experience.
Each level of experience increases the combat stats of the troops – attack, defense, damage, hit points, and speed; archers also increase their ammunition. To see how many bonuses a specific unit will receive, double-left-click to open its window and right-click on the image; the result will be approximately as follows:

This table should be enough to understand how it all works. From it, you can derive an important consequence for practical tasks – how much percentage of total power a specific experience level adds, in other words, how to convert an upgraded stack into its non-upgraded equivalent.
Let's consider an unupgraded unit in a battle against troops of its level as an example.
At level 1, the unit gains 1 health (rounded up) and 1 defense. That is, 100 level 1 units are approximately equivalent to sqrt(1.1/(1-0.025)) ~ 106 units without experience. By calculating all levels in a similar way, you can get the following table:
0 - 100
1 - 106
2 - 109
3 - 127
4 - 139
5 - 147
6 - 150
7 - 168
8 - 180
9 - 189
10 - 193
As you can see, at level 10, the units almost double their combat value.
The formula does not take into account the speed bonus – starting from level 7, units gain +1 to speed.
If you create a similar table for angels, you will get approximately the following:
0 - 100
1 - 107
2 - 115
3 - 123
4 - 131
5 - 140
6 - 149
7 - 159
8 - 169
9 - 179
10 - 190
Similar tables can be created for other units, and they will all have a similar structure.
For lower-level units, efficiency increases unevenly – the levels that increase damage are of great importance. For higher-level units, the dependence of strength on the level is almost linear.
I hope this small material will help you better calculate the strength of your army when playing with experience.
So, when playing with experience, troops gain levels during battles, from level 0 (no experience) to level 10 (elite).
Newly purchased troops have zero experience. Experience is retained when upgrading. When merging two stacks of the same type into one, the experience of the resulting stack is calculated based on the principle of preserving the total amount of experience. For example, if one stack has 20 units with 10,000 experience, and the other has 60 units with 0 experience, the result will be 80 units with 2,500 experience.
In battles, all troops in the player's kingdom gain experience – not only those in the battle, but also those roaming around or stationed in the castle (recruited). This is what makes experience valuable – it encourages players to engage in as many battles as possible, fight on multiple fronts, actively use roaming heroes, and generally play more aggressively than when playing without experience, where such actions often only increase losses and time spent without a significant benefit.
Experience is only awarded for won battles – experience is not gained when retreating or paying off the enemy. Experience is also not awarded if the battle is won by the castle/mine garrison in the absence of a hero.
Troops gain experience equal to 50% of the experience gained by the hero. The Learning skill only affects the hero's experience and does not affect the troops' experience – they will gain the same amount as without Learning.
When killing upgraded enemy troops, experience is awarded based on their nominal health, without taking into account upgrades or the 25% bonus of special units.
Each troop level has its own experience scale required to reach that level. Lower-level troops level up faster, while higher-level troops level up slower. For example, level 1 troops gain their first promotion at 1000 experience, while level 7 troops gain it at 4000 experience.
Each level of experience increases the combat stats of the troops – attack, defense, damage, hit points, and speed; archers also increase their ammunition. To see how many bonuses a specific unit will receive, double-left-click to open its window and right-click on the image; the result will be approximately as follows:

This table should be enough to understand how it all works. From it, you can derive an important consequence for practical tasks – how much percentage of total power a specific experience level adds, in other words, how to convert an upgraded stack into its non-upgraded equivalent.
Let's consider an unupgraded unit in a battle against troops of its level as an example.
At level 1, the unit gains 1 health (rounded up) and 1 defense. That is, 100 level 1 units are approximately equivalent to sqrt(1.1/(1-0.025)) ~ 106 units without experience. By calculating all levels in a similar way, you can get the following table:
0 - 100
1 - 106
2 - 109
3 - 127
4 - 139
5 - 147
6 - 150
7 - 168
8 - 180
9 - 189
10 - 193
As you can see, at level 10, the units almost double their combat value.
The formula does not take into account the speed bonus – starting from level 7, units gain +1 to speed.
If you create a similar table for angels, you will get approximately the following:
0 - 100
1 - 107
2 - 115
3 - 123
4 - 131
5 - 140
6 - 149
7 - 159
8 - 169
9 - 179
10 - 190
Similar tables can be created for other units, and they will all have a similar structure.
For lower-level units, efficiency increases unevenly – the levels that increase damage are of great importance. For higher-level units, the dependence of strength on the level is almost linear.
I hope this small material will help you better calculate the strength of your army when playing with experience.