The Spearfighter Barracks is the first military building in the game, which trains the very first light units - Spearfighter. It is required that the player to have researched the Spears technology before able to build it.
Researching the Spears technology and building a Spearfighter Barracks is part of the tutorial. The Spearfighter is most effective against fast units. It also has an additional defense when standing above tiles with bushes and forests.
Like most military buildings, the first two slots are free, with two others sold for coins and supplies, and one for diamonds. Unlike others, though, each spearfighter is trained in only 20 seconds.