By using a level 8 Mirror, you can mirror cards 1 level higher than its level cap.
Deploying cards costs Elixir, which is gradually gained during battle.
Cards can be obtained through buying them with Gold in the Shop or finding them in assorted Chests.
Chests can always contain cards from Arenas that you have already passed (except the Legendary Chest which contains a Legendary card from any arena).
Unlocking a new arena adds new cards to the Chest rewards - none are taken out.
If you received Chests in a lower Arena, it will only contain cards from that arena and below, even if you advance to a higher arena.
This includes Free Chests and the Crown Chests which contain cards from the arena you earned them in, even if you got the crowns in a lower or higher arena.
Once you have unlocked a Card, you don't have to unlock it ever again. Even if you donate your only copy of an unlocked card, you get to use the Card permanently.
Additional cards obtained will fill up an upgrade meter.
Once the meter is full, you can upgrade the card with Gold. Upgrading the card occurs instantly.
If your card has reached the max level, you can hold additional cards (100 for Commons, 10 for Rares, 1 for Epic and Legendary) for donations.
When even this meter is maxed out, each card received will give you a certain amount of Gold. This also holds if you have enough cards to upgrade to the max level but haven't upgraded it yet. For example, when the upgrade meter of a level 10 Common card reaches "8100/1000", each additional card will be converted to 5 Gold.
Every Common over the maximum gives 5 Gold.
Every Rare gives 50 Gold.
Every Epic gives 500 Gold.
Every Legendary gives 20,000 Gold.
Card levels in combat are displayed next to their health bar, with the exception of spells.
However, your cards will not display their levels if undamaged.
That being said, the player's cards do temporarily show their levels while being undamaged if they are under the effect of an enemy spell that deals no damage, such as Freeze.
For cards, all health and damage stats are increased by roughly 10% per level.
Cards that are able to spawn troops, such as the Goblin Hut and Witch will have their spawned troop level increased by 1 per level.
The level cap of Legendaries in Friendly Battles is Level 1.
On 3/5/16, the May Update increased the max level of King's Tower to 13 (from 12), common cards to 13 (from 12), the max level of rare cards to 11 (from 10), decreased the max level of legendary cards to 5 (from 6) and reduced the amount of epic cards needed to upgrade from level 7 to level 8 to 200 cards (from 300). It also added 6 new cards (Fire Spirits, Furnace, Guards, Lava Hound, Miner, and Sparky).
This means that there are new caps for Friendly Battles (and Tournaments):
King's Tower: 9 (from 8)
Common: 9 (from 8)
Rare: 7 (from 6)
Epic: 4 (from 3)
Legendary: 1 (unchanged)
All the players that had already upgraded their Epics or Legendaries to max Level received Gem refunds for every card over the cap.
You are guaranteed to get 1 Epic during the tutorial.
The chest you get is based on the arena in which you battle in, not your current one. You will always fight in the higher arena (Example: if a player from Arena 5 is matched with a player from Arena 4, then they will fight in Spell Valley, which is Arena 5). This means that you can get cards from an arena that you have not reached yet.