Is there any red surrounding the green movement area? I'm surprised the senate did'nt get on your butt for being on allies land without access rights. My guess is the retreat pinned them in between the fort and the sea but if you are allies you can go straight through there troops but not there cities. Only if you are at war or are neutral to them do you have to go around there armies. So my guess is the city itself has you pinned in so the only way out is by ship or taking the city.

As far as access rights, you can go anywhere you want but remember it is not taken kindly for violatiing foreigners lands without access rights. And if you do'nt get those troops out soon the Gauls will be offended and go back to war with you.