I have come across the same situation also. I was attempting to hold onto a Gaullic City that I had just taken, when a large Gaullic force (lead by thier faction leader) besiged the city. I sent some light cavalry out to try to thin the Gauls, as defeat was inevitable, or so I thought. I mangaged to destoy thier siege equipement, lost all my cavalry and fought them to a draw (aka hid in the city till the timer ran out). Then, the Gauls sieged again, but with no cavalry to take out thier equipment I resigned myself to defeat in 5 turns, but then they just up and left, the reason for which still escapes me.