I had a pretty difficult time with the initial siege part. Here's some tips that got me through it:
* There is a checkpoint in this fight, and if you get past it then fail, you'll start at the checkpoint with full roman morale/life/whatever.
* The siege shots are pretty easy to avoid, so I always had the archers assigned to stop the towers.
* Move back and forth from each side as much as you reasonably can, kicking down ladders as you can, it does help.
* ABUSE YOUR ARCHER SUPPORT. The will almost never kill anyone, but they will get enemies low enough to be executed, and if you group your enemies right you can nearly always get a double execution, which is great for getting your health back.
* ABUSE THE UNLIMITED PILAS/SPEARS. In each corner of the back wall there is an endless supply of them. These are great for destroying shields, or dropping an enemy low enough to execute. Anyone who be executed can also be killed with a quick throw (
), and it's just as important to keep your squad alive as it is to keep yourself.