Spear fighters, aka spearmen, are the most basic infantry units. They have low defensive and offensive capabilities, but are good at attacking calvary and have the highest haul capacity amongst infantry units (if paladins are not considered to be infantry).
Occasionally, Spearmen have a tendency to trip and fall on their own spears. This can be noticed when you send them to attack a village that has no troops, they somehow still manage to die.
Early Game Strategy Edit
Early in the world, spear fighters are going to be extremely important because they are the first units you build, they are good farmers, and they are cheap. I suggest having a good number of spear fighters for the first week or so, or until you research Light Cavalry, as you will need them to farm. The more spear fighters you have, the more you can farm. The more you farm, the more resources you have, the more resources you have the more you win.
Later Game Strategy Edit
As the game progresses, spear fighters sustain their importance to the player and Light Cavalry, do not drop out of existence. Though you can still use them to farm, their main use later in the game as one's character gains more points, is primarily to defend you villages(s) against Cavalry attacks. Spear fighters now become your most vital defense; arguably as important as swordsmen. Most people suggest building spear fighters in a 1:1 ratio with swordsmen.
Though spear fighters have several advantages, they do have a few disadvantages. The largest problem with spear fighters is their lack of attack power. Early in the game, if you are farming even a semi-active player and he/she upgrades their wall even a few levels, they can begin killing your spear fighters even without a defense of their own. Furthermore, if you are farming with them, and the person gets support, they will die in large numbers. Therefore you should never use spear fighters to attack someone you know has an army, and use them only to farm inactive players, because countering them is easy.