Assassin's Creed II/Sequence 12: Forlì Under Attack

In addition to normal memories, this sequence has a "Special Memory", located at the top of the tower in the northernmost part of Romagna where you can use the contraption from Sequence 8 to fly around. This isn't part of the story, and in fact when you begin flying there will suddenly be fires all around the map to keep you afloat with no explanation as to why or how they were started. In effect, because this memory can be accessed at any time and any number of times, it allows you to get the Fly Swatter achievement or trophy without having to completely restart the game if you missed it in Sequence 8.

For the PlayStation 3 and Xbox 360, this DNA sequence must be downloaded separately to be played. If you finish the game before getting this DLC, you will go straight to Sequence 14 from Sequence 11. If you already have the DLC installed, you must complete it before moving on, just like any other sequence. If you don't have the DLC, the in-game reason for its absence is due to a corrupted data stream, though Shaun will give you some info on what went on during the missing years. When you install the DLC after having already completed the game, Rebecca will inform you that she's repaired the DNA sequence.

A warm welcome
Walk with Caterina and Niccolò Machiavelli down the road to Forlì, until you find that the Orsi brothers have been sent by Rodrigo Borgia. Soon after you start running to the gates of the city, you'll be attacked by a small contingent of enemy soldiers. Take them out, starting with whoever Caterina is fighting, since she doesn't have armor. Afterward, keep running to the front gate where you'll enter a short cut scene. You'll become notorious and be tasked with entering the city to circumvent the locked gates. You'll find a gate you can swim under in the wall of the city to the west and slightly south of the main gate. Instead of diving right into the water, run along the edge until you can jump into a little boat, then row over to other entrance, past the dock where the men are fighting. When you get to the gate in the wall, get out of the boat and hold the legs button to dive and swim under it.
 * Memory start: Talk to Caterina Sforza on the road southwest of the Avamposto Veneziano.
 * Reward: 2,000ƒ

When you're inside, your new objective will be to open the main gates for your friends. Run along the inside of the outer wall to the northeast, and just before you get to the stairs leading up in front of the gate, climb the crates on the right side and go up to the roof. From there you can take some wires across to the top of the city wall, then over to the tower rooftop just above the gate where the mechanism is. If they haven't noticed you yet, air assassinate the two archers up there (fight them if they have), then interact with the gate mechanism to let Caterina and Niccolò into Forlì.

Bodyguard
You'll make a leap of faith from the top of the city gate into the hay cart below, after which you'll need to help your friends defend against some enemy soldiers. When you've dispatched them all, follow Niccolò and Caterina south, farther into the city. They won't stop to fight anyone, but as soon as an enemy's status goes to full alert on seeing you, they won't move until you're incognito again. Keep following them to the Rocca di Ravaldino, where a large group of bad guys is waiting. Once you take out the initial group, you'll be ambushed again by more enemies. Defeat them as well, then head into the white marker at the entrance to the Caterina's citadel.
 * Memory start: Automatic
 * Reward: 2,000ƒ

Holding the fort
Inside the citadel you're under attack from the moment the memory starts. You'll see white markers in different places, but these are to refill your ammo, rather than end the memory. Caterina is down in the courtyard with you, and Niccolò is up on the walls fighting. Take out everyone on the ground first to protect Caterina, then follow her up to the top of the walls. Whenever you're done killing the guys in any one place, follow Caterina as she jumps to the next tower top. If you don't stay near her, she's liable to fall off a tower, and when one of your allies dies, the memory will desynchronize.
 * Memory start: Automatic
 * Reward: 4,000ƒ

Godfather
The Orsi brothers have Caterina's children, and you'll need to retrieve them before time runs out. When the memory starts, you'll have ten minutes to get the kids back, and the first one will be to the north, outside the city walls. The quickest way over there is to head directly west across the rooftops of the citadel and jump to road outside the walls. When you turn north, you should be able to get a horse to make the trip to the first child much quicker. Head up the road over first one bridge, then turn east with the road and go over another one to enter the small homes where Bianca is being held. You'll find her being guarded by two soldiers next to a building by the southern bridge. You can double-assassinate those two from above, but it will be hard to stay incognito if you don't take out the archer one rooftop over and avoid all the roving patrols. Whether you're sneaky enough, or you end up fighting everyone, you'll need to have no alert status to talk to Bianca for the rescue.
 * Memory start: Talk to Caterina at the top of one of the towers at the Rocca di Ravaldino.
 * Reward: 4,000ƒ

The next child is being held at the top of a tower to the southeast, and is guarded by Ludovico Orsi himself. Take the horse you rode in on or the next closest one over to the tower. Come in from the north and get off your horse for a stealthier approach before entering the restricted area. You'll see an archer on a low rooftop just north of the tower – take him out first, then climb up the broken sections below the tower (you should see some crates with a white sheet on them you can free run on). Either assassinate the guard on this section or wait for him to go back downstairs before starting to climb up the tower via the wooden beams sticking out the side. When you get up to the section with the fire, watch for the guards walking around the path and be aware that they're not always close enough for a ledge assassination. When you can, climb the rest of the way to the top of the tower, then jump up and assassinate Ludovico when he turns his back to you. To end the memory, speak with Ottaviano.

Checcomate
Your job is to find and kill Checco Orsi before he can escape to the mountains. He's right outside the southern gate, but you can sneak around behind him if you jump over the wall at the back of the citadel. If you're sneaking up on him from behind, just climb the roof of the building he's next to, then jump down for an easy air assassination. If you want to come out the southern gate, he'll take off running as soon as he sees you. Don't bother with the soldiers that will attack, just jump on a horse and gallop after him. When you catch up to Checco, jump off the horse and take him out.
 * Memory start: Enter the white marker in the courtyard outside the Rocca di Ravaldino.
 * Reward: 7,000ƒ, Codex map

Far from the tree
You're looking for someone who can give you information on the black-robed monk with a missing finger that stole the Apple of Eden from you. He's on the back (northwest) side of the abbey. There are soldiers patrolling the whole restricted area, so make sure you stay behind them, out of sight as they move around. From where you start the memory, if you face the abbey you should see a roving patrol up the road ahead of you, as well as one farther off to the right. It's pretty easy to run in behind these guys as they're walking away from you, first the near patrol, then the far one, until you get to a small walled-off area at the back of the abbey. Run in and quickly assassinate the two guards giving the monk grief, or climb one of the walls above them for an air assassination. After you've helped him out, the monk will tell you to talk to the abbot of the abbey inside Forlì.
 * Memory start: Enter the white marker at the start of the road leading up to the abbey north of the city.
 * Reward: 7,000ƒ

Head into the city and find the abbey, then enter it and approach the abbot (use eagle vision to spot him). When you get close enough, there will be a short cut scene, after which the abbot will take off. Be prepared and you can easily tackle him as soon as you regain control so you don't have to chase him at all.

Moving on
In console versions of the game, Sequence 12 and Sequence 13 are DLC which must be downloaded separately, while they are included as part of the full version in the Windows version. If you don't have the next sequence at this point, see X marks the spot at the beginning of Sequence 14 for the requirements to progress to the final DNA sequence.