Here is an historicaly trick I found few days ago when I tried again Mongolia in Monarch difficulty :
=> Start as 600AD scenario.
=> All is about to be fast. Fast conquering, but beware of gold "bankruptcy".
=> Try to have 2 GP before you collapse to launch a Golden Age ! It prevent from collapsing. I built GP on Qufu in China (3 super specialists) an Mukden in Manchuria ( 2 super specialists).
=> Build some new cities. 5-6 would be enough. Space them as much as you can. In Siberia and central Asia.
=> Rush the technology to build culture in cities (grab only the ship thingy technology you can research from start).
=> Try to grab some vassals and work your relations with some civs. I had khmers and vikings as vassals.
Maybe force some ennemies to surrender, like russians (not China, too early in game, they will collapse).
=> Of course, since the start, build roads to west and build more and more troops (Keshiks only) to conquer.
=> Have some cities in "wealth" is usefull to prevent 0(-X) gold.
=> Dont raze cities, of course, before reach the 12% world territory goal.
I had 12,60% of world territory and it's close to historical expansion.