The Alliance Showdown is a weekly passive event where all you have to do is Register.
In each round, two alliances compete against each other on three lanes. Each player can only register on one lane. Any number of players can register on each lane, but only the 20 strongest players of a lane will participate in the fight.
The fight is one against one. The troops that are defeated or have won twice are out. The lane is won by the alliance that has at least one player left at the end. The battle is won by the alliance that has won the most lanes.
Registration always starts on Mondays and ends on Wednesdays at 23:00 UTC.
To register, select the event and click on Register.
Then select a lane and click on Send troops…
and compose a march as usual and send it off. This will not occupy a march slot and the heroes will still be available.
Until the end of the registration, you can change/update the lane and the troop formation, e.g. if you activate skills or buffs for the trap.