Select cards to play from the bottom of the screen, each card has a resource cost listed at it's top.
You start with 2 of each kind of unit: Builders, Soldiers and Sorcerers. Each of these produces one associated resource per turn: Bricks, Weapons and Crystals.
Attacks sent against your opponent will be absorbed by any Fence value he currently has. Once the Fence is destroyed you will damage his Castle.
Win the game by bringing your opponent's castle value to zero, or by bringing your own castle value to 100 or greater.
Source: The Game Website.