You command the Blue faction, your base sits at the bottom left. Grow your economy, build barracks, train an army, and destroy the red enemy base at the top right to win.
Left-drag to box select your troops (single click selects one, Shift adds); right click issues orders; once a barracks is built, select it to train units.
Unit types: Infantry (circle - melee) / Archer (triangle - ranged) / Cavalry (diamond - fast melee). Lose every building and you are defeated.