Sword Infantry are one of the three types of Infantry that can be recruited from the Barracks. As one of the base units, they are immediately available from the Barracks as soon as the game begins.
Sword Infantry are known to carry a variety of hand weapons as well as shields, granted them a degree of defensive versatility. They are more resistant to the attacks of Archers than other Infantry Units.
It's always a good idea to send along a good amount of Sword Infantry to soak up damage that might be directed at other Units. Due to their versatility as defensive Units, they are suited for both Attack Missions as well as defending Settlements.