Swordsmen are a Gallic attack troop. They are cheap, have a fairly strong offense, and a weak defense. They can be built at the Barracks after being researched at the Academy once you have Academy 1 and Blacksmith 1.
Swordsmen are a fairly slow moving offensive unit, but are the fastest way for Gaul to build up attack power. They make an excellent escort for Rams and Trebuchets.