In the game, you can purchase aPremium for 1,3,6 or 12 months, which provides the following perks: bonus for Gold and Prestige gained in the battle, increased chance of finding artifacts and other items. In order to buy a Premium, click the bays sign on the upper left panel:
Select the desired package from the list in the pop-up window:
When hovering your mouse over the Premiumicon, you will be shown its description:
You can also additionally buy here any of the specified amounts of BG.