Keep a close eye on the leadership skills of your units inside your forts. If you find a unit's leadership skill has increased to 40 or above, consider moving him to his own fort with raw recruits. His leadership skill will increase faster as a General provided he has units under his control (the more units under his control the faster the leadership skill will increase).
Know your race's military capabilities (especially ranged attack and shields). The manual shows when and how each race acquires their extra abilities. Train your units up to at least that level before attacking anyone. You may get lucky and survive a battle, but in all likelihood, your units will be in no shape to fight again for quite a while.
A General's leadership skill (as a percentage) is added to the combat skill of a unit under his command if the General is within 10 spaces of the unit. This is often the difference between winning and losing a battle.
As noted above, your General must be within 10 spaces of a unit to add his/her leadership skill. Assign 2 guards to your General (select two units plus your King and press Ctrl + #) prior to entering battle. Move this group close enough to the fight to gain the bonus, but keep your General out of the main fray as much as possible. Let the guards pick up any enemy units near him. Keeping your General healthy increases his effect on combat skills and leadership skills once he has returned to the fort.
Add a military strategy here.