I just played a game on ridiculous/ridiculous, large map, 8 AIs. I got an alliance victory on turn 149, without ever going to war with anyone.
I used a standard race, a custom sovereign that did not influence my strategy much, and around turn 100, I only had 4 cities (3 towns, 1 citadel)
Any guesses about how I did it? (no cheating, as to exploits, your mileage may vary)