Sounds like the Battle of Alesia was a conventional siege, whereas Sadr City was, well, "urban warfare", a battle to weed out the rebels from lived-in neighborhoods.
The Battle of Alesia wasn't a conventional siege in the sense that the defenders weren't behind a wall to defend themselves from open attackers, but the attackers were behind the walls using it against the defenders. The wall is offensive in this context, as a type of blockade, instead of a defensive structure.
