This is the most exiting feature of clash royale. Clan battle are event based that happen every other weekend.
Clan battle are between two team from different clan. All four player are playing together to win more crown.
King Tower has two cannon, wider and have two kings. Its damage are similar to normal king tower. Arena tower and card level are similar to standard tournament level. Every win your clan receive clan battle win to clan battle chest. Its similar to clan chest but based on clan battle win. So you need to play as many games to contribute to your clan.