That sounds pretty clever---to encourage them to retake the towns, after you abandon them you might give the Julii map information to make sure they know that they're free for the taking. The main problem would be stopping them from making further incursions before they actually get around to building the stone walls; the AI is kind of stupid about doing that. Or even better, I suppose you could offer to give the two provinces to them in exchange for a ceasefire and trade rights, which would encourage them to not attack you for at least a few turns and make it more likely they'd improve the cities.