The Barracks is a room where you train your troops in the basics of melee, ranged, and spell-based combat. The training schematics in this room include the "whirly-blade-thing" and "whack-a-dwarf" models - built out of the finest metals in the Underworld they should require no upkeep once built - which will certainly hone your creatures skills to a considerable degree. Unfortunately, they will not reach their full potential simply practicing against inanimate targets and will require live foes to raise themselves to their full potential.
The Barracks is one of the rooms within the Vein of Wrath.
Upgrade
Get the revolutionary spinning-scimitar-slinger and revolving-target-wheel, which will train your higher level minions faster.