In 1390, the Celts pulled a doublecross and used a Right of Passage agreement to send a band of guerillas all the way to the undefended city of Knossos. Rome promptly retook the city and killed every Celtic guerilla in sight, capturing a band of Celtic workers. More importantly, Rome allied the entire world against the Celts, aiming for their destruction (which, in turn, would provide access to furs, the last luxury that Rome did not yet have).
[I'm starting to seriously consider seeing whether a domination victory will be possible without ever capturing a single city from an AI except for retaking cities an AI took from me. With another city founded in 1395, I'm up to 40% of the world's land area. If I can get the AIs to kill each other off and slip in Roman cities in between, domination may be possible - although I may have to use a strange kind of ROP rape to plant the last cities I need.]