The year changed from 1BC to 0AD and that is actually an enormous change...
Miracles are about to happen and the formation of a cross-shaped peninsula is one of these (historically not correct, but who can exactly know what happened so many years before --- People easily tend to forget things)
Caesar wants you to take over the mission to colonize this Peninsula. And the city has to be large and prosperous to reflect the miracle which managed to form such a disnatural looking event.
Oh, the Celts actually have an eye for crosses and this one you have to colonize seems to be some eyecandy for them, so make sure that you can expect some minor nuisseances here.