This is a free in-game currency which players acquire as they do certain tasks and achieve milestones. Different activities in the game will require spending a certain number of crystals. Apart from earning crystals by doing certain activities, their earnings are also time-based. Means, players earn crystals automatically for free with the passage of time irrespective of whether they play the game or not. There is a limit of 30 Crystals which a player can have at any given time. Crystals do not increase after they reach to 30 Crystals limit.
Crystals are used directly in battles with other players to raid/capture their Type B lands: For raiding lands, it costs 3 crystals, and for capturing 6 crystals. A player can raid directly on another player Garrison which costs 6 crystals.
However, the maximum default limit of Crystals can be increased with Crystal Tower. This is a building that stores Crystals, which are used to battle with enemies. Upgrading your Crystal tower increases the number of Crystals your Empire can generate. This building can be upgraded 4 times at most. And each upgrade increases the Crystal limit by 10 Crystals. Hence, the number of Crystals which can be owned at most in League of Empires game is 70 Crystals including the 4 upgrades of the Crystal Tower.