The Farm Defense

The Farm Defense in Action
Many people tend to overlook the most basic defensive unit in the game: the lowly farm. With 400 hit points and 20 armor, a farm can take a lot more damage than any unit, wall, or tower. They cost 500 gold and 250 lumber to build, which is about the same as a scout tower. More importantly, they're a necessary part of any town- you can't stage an offense without them. Since you're going to build farms anyway, don't hide them away in the corner- use them to block off possible attack points around your town. Back up the farms with towers and/or ranged units to pick off anyone who tries to break through.

To defend against magical attacks and catapults/ballistae, keep a small strike force of knights or ogres off to one side, outside the blockade. Only use them when you're attacked by units your towers or archers can't take out. Once your strike force has been upgraded to ogre-magi or paladins, they can be used to support your archers/axe throwers by healing or bloodlusting them. After each attack, pull a peon or peasant away from his normal duties and have him repair any damaged buildings.

The Farm Defense in Action
A group of ogres, normally kept away from the blockade, moves into action against a ballista.