Wow, sounds like you could even get a free fairly expensive city!
If you wanted to make them turn their stack around you could head straight for Rome (nb I'm definitely not suggesting that!) but I could see you keeping Arretium with enough catapults and axes in there... Have you simmed any cat+axe vs praet battles yet? Might be fun to see how many cats it takes to give you good odds.
Although am I right in thinking GG are saving promotions to heal with after getting collateraled? Meaning you'd need to attack out with the axes rather than sitting behind walls. Maybe keeping it isn't so great in that circumstance.
If you wanted to make them turn their stack around you could head straight for Rome (nb I'm definitely not suggesting that!) but I could see you keeping Arretium with enough catapults and axes in there... Have you simmed any cat+axe vs praet battles yet? Might be fun to see how many cats it takes to give you good odds.
Although am I right in thinking GG are saving promotions to heal with after getting collateraled? Meaning you'd need to attack out with the axes rather than sitting behind walls. Maybe keeping it isn't so great in that circumstance.

Completed: RB Demogame - Gillette, PBEM46, Pitboss 13, Pitboss 18, Pitboss 30, Pitboss 31, Pitboss 38, Pitboss 42, Pitboss 46, Pitboss 52 (Pindicator's game), Pitboss 57
In progress: Rimworld
In progress: Rimworld