Axe Throwers are expensive units but have an exceptional ability to decimate almost any troop. Their only downfall is their range, which is very short. While this can obviously be extended through the use of walls, it is still significantly smaller than that of Archers or all other ranged units in the game.
Axe throwers have an incredibly fast attack speed, and can quickly deal out a lot of damage to enemy units. To kill an Archer for instance, it only takes an Axe thrower a single throw of both axes. The Axe thrower is also terrible in melee combat in the instance that troops can actually get close enough to him to attack.
These units can be the ideal units to defend some of your estates from buildings placed around the flagpole, or your walls or Gatehouses. Should there be any narrow passes on your map, axe throwers would be the ideal unit to keep watch over them as they will decimate passing troops.
You can recruit an Axe Thrower from the rank of Knight Bachelor or higher. All you need is 100 gold, along with 1 peasant from the campfire.