Here is what the developer had to say about it,
In this video, we take a look at some of the great new features we’ve implemented into siege battles in Total War: ATTILA, set against the backdrop of Londinium in 395AD.
Dynamic fire simulation: Fires can break out in a city as a result of burning projectiles or marauding soldiers. They also have a chance of spreading from building to building, according to the weather conditions. The city’s defenders become affected as it sustains increasing levels of damage, and will experience growing penalties to their attack, defence and morale characteristics.
Multiple siege escalation states: If you attack a city directly without besieging it on the campaign map, you’ll be faced with undamaged walls and stout defences. However, conduct a siege for several months beforehand, and you’ll gradually wear down the city’s defences to the point where the walls become breached, and fires may already be sweeping through the city when the battle is initiated.
Barricade deployment: Defenders may now erect barricades at the start of a city battle. These are destructible, but provide a temporary defence to any units mounted upon them. However, they take time to break through, so enemies may opt to avoid them, which gives the player the opportunity to funnel invaders around the city into their own bottlenecks and killzones.
Take a look at the new video below:
Total War: Attila is slated for a 2015 release for PC.