Skills

Note: it is not explicitly named in the game otherwise than "upgrades"

Skills can be upgraded in the library.

You can earn experience from multiple sources :
 * during battles (even in defeats)
 * during PvP combats
 * by opening chests as Clan events reward
 * by opening chests on the map using portal keys

Each time you pass a certain amount of experience you will earn a skill point. Each skill can be upgraded 30 times. To maximize everything, you need a total of 690 points.

There are 3 tiers of skills which you can unlock at some tower levels.

Tier 2
Unlocked at tower level 80.

Tier 3
Unlocked at tower level 150.