Massilia is a veritable fortress. It is perhaps the most well defended settlement in the game. There is only a single entrance into Massilia by land and that is over a single bridge. Very conveniantly, this bridge also overlooks the entrance to the weatern mountain pass. This means that if any army wanted to go through the western mountain pass (the only mountain pass that the gauls have access to) they would first have to get by any army stationed on that bridge.

This means that even if they didnt want to, the gauls would have to fight a bridge battle on the offensive and ourselves on the defensive to get passed us. It means that northern italy will never have to worry about Gauls gettig passed the alps. Its basically a fool proof defense system that would be nearly impossible to break. Such an opportunity just cant be passed up as i doubt there is another settlement in the game that rivals Massilia in its defensive capabilities.