Hero gains experience (XP) in battles, from adventures, daily quests, items and tasks.
Once the hero reaches enough experience, they gain a level and can spend extra attribute points.
Getting XP in battles
The amount of XP gained in battles is directly connected to the enemy army's losses. Each point of supply of the killed unit grants one XP.
In offensive battles this is simple, if your hero was with the army, they receive the full amount of XP from that battle for all killed enemy units, even if the battle was lost and even if the hero didn't survive it.
In defence battles, this is complicated a bit due to reinforcements. There could be multiple heroes in defence, as well as other units from multiple villages. Due to this, XP granted to defending heroes is divided based on the proportion of each hero's army to the total defence, counted in the units supply points. This is similar to how points for the defence ranking are calculated.
Example 1
An alliance defends a target in the early game with heroes only. There are no other defending troops involved. Each hero gets his/her fair share of experience.
Situation: Player A attacks with 10 Theutates Thunders and 10 Phalanxes = 30 crop consumptions
Assuming 10 heroes are defending, and all attacking troops are killed. Because the supply is the same for each hero, (6 crop), it means that each hero will get 10% experience out of the amount of total killed supplies; as a result, every hero will get 3 experience points.
Example 2
At an alliance defence-operation, one player sends his whole anvil, including the hero, while another player only provides his hero. The player providing more supply than just the hero will get a bigger share of experience.
Situation: Player A attacks with 10 Theutates Thunders and 10 Phalanxes = 30 crop consumptions
Assuming 2 heroes are defending; of which one defends together 88 Phalanxes (100 crop supply in total). If all attacking troops get killed, the experience will be shared differently. The solo hero will get 6% of the experience share, and the other hero which was sent with the phalanxes will get 94% of the experience. This means the solo hero will get 2 experience points, and the hero with the phalanxes will get 28 experience points. We use mathematical rounding.