The 6 vs 6 battleground was not playable on the CBT. Due to his I can only provide you with confirmed basic information.
Once the battleground is released to the public the information will be updated.

You have to destroy the enemies guardian stone to win the battleground.
Every room is filled with neutral creeps.
Neutral Creeps give XP.
XP will increase your Warlevel.
Your Warlevel will increase your overall stats.
Your Warlevel is personal based – not team based.
The rooms have door that periodically open and close.