Head down the slope after Arno goes underneath the tree branch. There is some danger in this area as there will be cannonballs and the like flying around. Keep on the move and try to stay fairly close to cover.
Go down the slope. As you approach the wooden barricade, move close to it to avoid and incoming cannon ball.
Head around the first corner and turn to the left. Follow the trench over to the left and then head for the next wall. Keep the wall to your right as you start toward the final rush toward the portal itself.
Keep to the left until the cannonball hits. After that, switch over to the right-hand side of the bridge. Just make the rush toward the portal and jump into it.