Oh wow, I never even thought of doing this, sounds like a pretty fun sandbox game. You just start a game out with no other players? Do you have to disable diplomatic victories and such? I imagine you'd have to really crank up the difficulty to make it challenging?
edit: Oooh, and have you tried spawning off vassal colonies in these games?
Me vs the Raging Barbarians is a very fun game. I play with some basic rules to liven things up:
Only victory enabled is domination.
Size small or larger.
Always a land mass map (inland sea, pangea, or fantasy). I believe this gives me a disadvantage more than the barbarians.
No building the great wall (that's cheating!)
No building spies.
I play Monarch/Emperor on normal civ games (I'm about to try my first push into Immortal this weekend) and I usually play this type of a game on Monarch/Immortal. That's about a half to one level above normal play. I tried a Deity raging barbarian game last night as Montezuma (starts with hunting) and this is how I played my cards:
Turn 1 Moved one space west to found my capitol on a hill.
Turn 1 Sent my scout out scouting.
Turn 2 Started a Barracks
Turn 12 Hinduism has been founded in a distant land. Holy Barbarians!
Turn X (less than 20) I discovered Archery.
Turn X I changed from the partially built Barracks to a bowman.
Turn 25 A Barbarian archer approached my glorious empire.
Turn 25 Swapped city to full on production and started moving one of my 3 scouts (I popped 2) back towards the capital.
My closest scout missed making it back to the capital by ONE turn! And I was going to miss building an archer in the city by one turn, too! My scout's last move, in an act of desperation, was to a plains tile next to the barbarian archer in an attempt to lure it to attack my scout giving me that extra turn to build that archer in city on top of hills. But alas, my plan went over like a fart in church. The barbarians ignored my scout and took over my city.
Turn 30 You have been defeated!
Deity vs Barbarians is rough. I died on turn 30! Oh well, I learned something about the first 30 turns. (Build a Warrior first, THEN an archer.)
P.S. I've never thought about a colony. That seems like it wouldn't work all too well, however. If I recall correctly, you only receive 1/2 the bonus land for a domination victory. The barbarians would probably rip them apart, too, further exhausting the military I would need to produce. And it seems a bit like cheating in a "one player only" game.