Assassin's Creed II/Sequence 4: The Pazzi Conspiracy

This sequence will teach you a host of new skills, as well as introducing you to your first Assassin's tomb. You'll also make two powerful allies here, a notorious thief and benevolent politician.

Practice what you preach
Leonardo will translate the Codex page you got from Vieri, and he'll be able to upgrade your hidden blade. In the meantime, you'll need to practice the new assassination techniques on the dummies set up outside the workshop. Head into the courtyard adjacent to the workshop to get started (you'll see three red target indicators on your map). You can perform the assassinations in any order, and as many times as you like: You can practice on the dummies as long as you want, and when you're done just head back into the workshop. Leonardo will give you back your hidden blade with the improvements he's made and he'll tell you to talk with La Volpe to find a way to your next target.
 * Memory start: Knock on the door to Leonardo's workshop.
 * Reward: 1,000ƒ, Air assassination skill, Assassinate from hiding skill, Second Hidden Blade, Races unlocked, Assassinate from ledge skill, Hidden Blade armor plate
 * 1) Hide in the hay cart and press the armed hand button to learn "Assassinate from a hiding spot".
 * 2) Climb up to the railing in front of the dummy high above and press the armed hand button when you're in front of him to learn "Assassinate from ledge".
 * 3) Climb up to the ledge above the target dummy in the back of the courtyard and press the armed hand button to learn "Air assassinate".

Fox hunt
When the mission starts, you'll see a circular area marked in green on the map, which means your target is located somewhere within. Usually you'll need to use eagle vision to locate your target in gold among the people present, but for this memory, you just need to get close to him. Head around to the opposite side of the market and you'll find man standing alone by one of the pillars on the end. When you get close enough to him, a cut scene will trigger with a thief picking your pocket and running away. Run after him and tackle him by pressing the unarmed hand button when you get close enough. He runs pretty slowly compared to you, so this shouldn't be a problem. When you've caught him, you'll see a cut scene where you meet La Volpe and ask him to help you find Francesco de' Pazzi.
 * Memory start: Enter the white marker outside the Mercato Vecchio.
 * Reward: 1,500ƒ

See you there
Follow La Volpe as he runs through the city to Santa Maria Novella. Start out by climbing up the same way he went to the first white marker. After that, you'll have one minute to get across the district to your destination. It's a fairly direct shot across the rooftops; just remember to climb up to a higher roof when it looks like you'll have to jump down or go a different way. Eventually you'll end up on a rooftop next to Santa Maria Novella, where La Volpe will teach you how to get into secret catacombs. This skill is used to enter Assassin tombs and collect seals to unlock Altaïr's armor back in the Sanctuary at your villa, but the first one is part of the next memory.
 * Memory start: Talk to La Volpe at the top of the steps to the dock, just below where the last memory ended.
 * Reward: 1,200ƒ

Novella's secret
Don't bother distracting the guards to get into the area with the secret entrance – just climb the wall right beside them to get in. You'll find yourself in the catacombs beneath the Santa Maria Novella. This is a restricted area, as are all Assassin tombs, which means you're automatically notorious and guards will attack on sight. Head down the steps in front of you and pull the lever on the right to lower the block at the far side of the room. Climb up and out of the opening this created, and go through the next room out to a staircase and a cut scene showing you how to proceed.
 * Memory start: At the secret entrance in the guarded corridor on the south side of Santa Maria Novella.
 * Reward: 1,250ƒ, Seal of Darius,

Proceed directly ahead down the broken staircase and free run to a swing across to a beam and over to another ledge. Keep free running in that direction over some ascending beams that end with a swing to a final wooden beam over a wooden platform. From there you can jump to the ledge up above and pull the lever up there. This will raise the gate in the hallway inside the structure in the middle of the area, so jump over to it and run through to the ledge on the other side. Over there you'll be above the parts you were free running earlier, and you can head across some more beams and swing bars in the same clockwise direction to a final ledge with another lever. This retracts some wooden platforms that were blocking your way down to the haystack far below, allowing you to perform a leap of faith down the opening behind the metal gate in the large pillar in the center of the area.

When you get down there, exit the haystack and walk to the hallway leading to the guards. A cut scene will play, showing the guards walking into the Great Hall and a lever you need to pull to open the door they went through. When you enter the area, you'll see a pool of water between you and the door. Look to the left and you should see the telltale white sheet on a low broken wall, signaling the place where you should start your free run. Jump across the wall tops to the wall behind them and then shimmy across to the left. This will put you at a ledge leading up to another area. Wait here for the guard to come by, and pull him over the edge when you see the "Assassinate" command on your armed hand button.
 * The door to the Great Hall

Climb up and follow the stairs there to next level where you can assassinate the other guard. At the far end of of where the guard was patrolling, you can free run across a pole and a couple beams to a swing around a corner that will drop you right next to the first lever you need. A secret area with a treasure chest is located in the hole in the floor just beyond the lever. Pull the lever and head over to the beam on the other side of the hole, where you can run up the wall to a wooden platform then jump over to a small balcony with a wooden beam sticking out of it. From here, jump over to the wooden coffin you raised with the lever, then turn to the left and jump to the pole which leads to another platform. This puts you right next to the area with the lever that opens the door to the Great Hall.

When the door opens, three of the four guards that left earlier come back in to investigate. They all patrol randomly around the pool of water on the ground floor, although the captain goes back behind the walls from time to time. From the lever, head to the right back the way you came (if you're facing the door you just opened), and jump over to the wooden platform on the far wall below you. From here you can get down to the area where the first lever is, and then it's just a matter of waiting for the captain to walk back away from the open area so you can air assassinate him. From here, just run over to the haystack by the wall next to the door where you originally entered this area and hide there. If no one saw you, you can wait for the guard that stops there to come around and assassinate him, then take out the last guy when his back is to you. If you are spotted, but your cover wasn't fully blown, things will go much quicker, as they come to investigate and you can assassinate them from the haystack when they come near. It's all right to just fight these guys, as the last guard won't be alerted unless you get close to him.
 * The first three guards

Your throwing knives have been disabled in this memory, so there's no way to not alert the guard outside. Walk over to the Great Hall and trigger the cut scene, after which you'll have to chase the guard down. The chase sequence is scripted, so no matter how fast or slow you go, the guard will stay ahead of you until the last hallway leading to the guard room. As you follow him, walls crumble and he closes gates, forcing you to find alternate routes. When a wall crumbles, keep going the same way he did and you'll just climb. When he closes a gate in front of you though, you'll need to look to the right or left for a way to proceed. They should be pretty obvious, as there's only ever one way to go. When you get to the final hallway, you'll see a ledge running along left wall above where the guard runs. Hit the beginning of the steps up to the ledge at full sprint and run along the ledge. At the very end, press your armed hand button and you'll do an air assassination. Note that the HUD doesn't display the normal "Assassinate" command, so don't wait for that.
 * Chasing the last guard

If you got the guard before he made it to the guard room, you can go right in and run up the wall to the secret door on the wall. If you didn't catch him, however, he shuts the gate and alerts the guards in the next room. In that case, you'll need to climb up through the opening above and to the right of the door and jump down for a fight against all of them at once. Your first air assassination will take out two, and after that just use the normal block and counter strategy to finish the rest. In either case, head through the door up on the wall and go down the stairs behind it. You'll see a cut scene showing the meeting between the Templars. Afterward, keep going down the hallway to a set of steps down to the tomb and the Assassin's sarcophagus with some treasure chests surrounding it. When you open it up, you'll find your first seal. Exit through the door highlighted on the wall.

Wolves in sheep's clothing
Head over to the courtyard just east of you (marked with a green circle on your map) and mingle with the crowd standing around there. You'll see a cut scene showing the Medici family arriving, after which they are attacked by Francesco de' Pazzi and his Templar friends. Giuliano gets taken out rather quickly, but his brother Lorenzo is still fighting, and it's him you'll need to protect. There are a bunch of guards and Pazzi, but unfortunately you can't kill him – when his health bar is depleted he just runs away. Finish off the rest of the guards with Lorenzo by either assassinating the ones that aren't paying attention to you or by simply fighting normally. After they're down, you'll have to take Lorenzo back to his home, which is off to the northwest. You'll have to take a circuitous around to it, as more direct routes have been blocked off with fiery obstructions. Don't bother to fight anyone on the way to Lorenzo's place, just run straight there. When you get there, kill off the guards waiting in ambush and get Lorenzo to enter the marker in front of his door.
 * Memory start: Enter the marker on the opposite side of the courtyard from Leonardo's workshop.
 * Reward: 3,000ƒ

Farewell Francesco
There is fighting in the streets between the Medici supporters and the Pazzi supporters, and you need to make your way southeast to the Palazzo della Signoria to assassinate Francesco de' Pazzi. This is the building where your father was imprisoned before he was hung, and you need to get up to the marker on the roof of a building on the north side of the plaza in front of it. You'll see Francesco up on the Palazzo, and he'll yell down for the guards to kill you. You'll need to go up the same way as before, from the back side. There are three groups of guards on the first rooftop, and more on the higher one. On second rooftop of the structure, you'll find Francesco at the northwest corner.
 * Memory start: Talk to Poliziano (the guard right outside Lorenzo's home).
 * Reward: 2,000ƒ

Head up the northern wall, which is the closest side from the last marker. Don't bother to fight the guards, unless you want to. Once you get close enough to Francesco, a cut scene will be triggered where he will attempt to escape by jumping over the wall where he was standing. When you regain control, take a leap of faith off the same part to a hay cart down below. As you land, Francesco will take off running and you're supposed to chase him down through the city to assassinate him. If you're quick, you can target him as you're exiting the hay cart and quickly switch to throwing knives for a fast take down. If you end up chasing him, you'll find it's much like chasing a Borgia messenger. If you haven't done that yet, it just entails chasing after him while he climbs around rooftops. You can fail the memory by falling too far behind your target, but he runs around in a circle, so there's not too much chance of that happening. When you get close enough, assassinate him normally.