I've learned to jump the competetion as early as possible and make the continent an island. Also, use borders to pen the other empires into corners, then destroy them.
As a general rule, in the early part of the game, 6-8 archers will whack just about anybody. Usually, by the start of the middle ages I've destroyed the 2-3 other empires on my landmass, and that's with barbarians set to ravaging (which actually helps you more than hurts you, especially if you park a unit on each of your developed resources so they can't be pillaged). I've gone into the Roman empire and found 3 of its 5 cities taken over by barbarians (I usually play that you cannot raze cities because it very rarely happened).
I've not played where everyone is in a single landmass.