Directly below the research button (mobile) is the shortcut to Premium Purchases. There are 5 tabs to choose from ... I have only used the far right (red button with white arrow). The other 4 tabs require payments in US $s. The 5th tab takes you to a screen with 3 AdMob selections.
Top left will add 1000 gold for watching an advertisement. Of the three AdMob selections, this one I rarely use if ever.
Top right will add 40 gold per day (game day). Even though it doesn't look much, each advertisement adds another 40 gold per GameDay (approx 2-3 seconds real time). Currently I have over 10k gold per day coming in with Tribute set to zero across the board. I'll admit to get that income has taken hundreds of adds but, it's the reason why I rarely use the 1000 gold. I generate 10 times that amount every 2-3 seconds when the game is running. For a military campaign, run out of troops or gold, you are dead meat but with ample at your disposal, you will steamroll every state ... they simply cannot compete.
The third AdMob gives you gems. Gems reduce time (research time, march time) but they play a very important roll in gameplay particularly attacking and annexing states.