@Stravlon - Crowns are earned through Quests, Sieges, PvP and Achievements (yes, you're actually rewarded for unlocking Achievements
You can also exchange Gold for Crowns. Gold is earned through mining, looting, trading and as part of the reward to some quests.
Crowns are used to unlock research, speed up construction and buy items like Attack Protection or Elite Units.
Any units purchased give very little, if any, advantage over other players and are more for variety/options as you'll be matched with players of a similar strength in battle.
Crowns can by purchased, but there is no requirement to do so.