You are the knight at a castle gate, and a goblin horde is coming down the road. Hold the gate through twenty waves, calling soldiers out to fight beside you, and spend the coins you win on a stronger knight, gate and army.
Walk your knight left and right along the road. He fights on his own: his sword hits anything in reach, and standing still he shoots his bow. He can't walk through enemies, and he heals slowly once nothing has hit him for a few seconds.
Leadership is the gold bar that fills up during a wave. Spend it on the soldier cards under the road to call soldiers out of the gate. The number on each card is its Leadership cost, and a card needs a moment before it can be used again. Coins are separate: they buy things at the camp. Soldiers march out and fight whatever they meet.
A wave is won when every enemy in it is beaten. It is lost if the gate breaks or your knight falls.
Beaten enemies drop coins. Pick them up by walking over them or tapping them, before they fade. You keep your coins whether you win or lose.
Goblins come on foot and on wolves. Slingers throw stones over your soldiers. Brutes carry wooden shields. Cyclopes swing clubs that hit everyone in reach and knock them back. The tenth wave brings the Elder Cyclops, and he is not the last.
Between waves you visit the camp. Upgrade your sword, bow and armour, the gate, Leadership, and Training, which gets the soldier cards ready again sooner. Hire new soldiers as the waves go on, and train the ones you have. If a wave is too hard, replay an earlier one for coins.
The leaderboard ranks the furthest wave you have held.