If an enemy king is exposed, use the Space Bar to split your army. This allows you to surround them or keep your own King safe while your troops engage.
Most players ignore the dark areas on the map. Hide a small squad of archers in the fog near a contested tower. When the enemy moves in to capture it, ambush them. This psychological warfare causes panic and misclicks.
The ultimate power-up, but expensive. Don't buy one until you have a solid wall of smaller units to protect it. The "Split" Tactic: