Fight against the enemy generals. You must win four battles and then fight one of several options for the last boss.
You can select initially among swordsmen, cavalry, archers and spearmen. As you play you can unlock more powerful units to add to your set of ten units. Trade them out and adjust your army as you see fit. Units are better or worse depending on who they go up against. eventually you can unlock specialty units such as monsters and ninjas.
Network multiplayer is unlocked when you defeat a boss.