In campaign mode you can start a single player adventure and you can start conquering lands with your army. Remember that strategy matters more than soldier traits. For example, you can place soldiers with low defense in the back like archers, and you should put soldiers with high defense in the front like knights. After you destroy every enemy unit on the field, you will win the battle.
Don't forget to try the game in two-player mode against a friend. Call a friend and challenge your strategies on the battlefields! If you play in the mode 2 players, both players set their strategy and then the game starts simulating their war and the best plan will win.
control
As for controls, use mouse to interact, X to remove soldiers, CTRL to place soldiers properly, right click to rotate camera, WASD to move camera.