Problem with attacking fullstack in minor settlement

Playing as the Romani I have just attacked a fullstack of Carthage's in a minor settlement. I didn't want to cheat (as per the these are not bugs thread) and so went on the offensive. Unfortunately all that the full stack does is not array itself in any sort of battle formation bu clumps together in a great square... There is simply no way of combating that. Flanking, fully surrounding does nothing because, being a 'settlement square' even when a unit breaks (because they have strayed from the square) they immediately reform, and wavering units that stay in the square will never break.

I'm going to try lifting the siege and hang about nearby and hope to draw them out. Otherwise I am really enjoying, absolutely amazing work.