The Training Hall was named in the Village description when Hearthstone: Mercenaries was first announced. After some time, the Training Hall was introduced as the Training Grounds.
The Training Grounds allows players to train Mercenaries passively. These Mercenaries gain experience over time, regardless of what the player is doing and even when the player is logged out. Players only need to check on the Mercenaries to claim their accrued experience periodically.
It has the same system as the Daycare in Pokemon, but players won’t receive eggs if they train two Mercenaries at once.
Training Grounds has only one prerequisite, to defeat Barak Kodobane in the Barrens. Once that is done, players are given the option to build The Training Grounds. The Training Grounds come with one slot and cost 100 Gold to build. Players can upgrade to add a second training slot for only 200 gold.
The Training Grounds will occupy the lower-left corner of the Village, which was previously just empty. It will appear as the training dummy seen above.
Are you interested in the new changes in Hearthstone? Check out Will Hearthstone be an Xbox exclusive? or All Hearthstone Battleground Buddies and their Heroes here on Pro Game Guides.