The level starts with you at the river. Use the white sheets to jump up the wall and onto the beams, jump across onto platform then look at wall for hand-holds and jump to grab them. Little bit of climbing next but pretty easy route to follow. There are three archers at top to fight, once defeated use switch to open door.
Another battle against three more enemies. You should be pretty good at getting timing for "counter" kills by now so take your time. Follow white sheet free-run to find switch to open door. Drop back down to where you fought most recent enemies and follow stairs up. More enemies here but don't be daunted by the numbers, just keep your right trigger held down to constantly parry and work away on your X-button counter moves. Once battle is won, climb parapet and jump down other side onto horse.
Gallop along wall on horse and attack enemies from horseback if possible, don't worry if you are knocked from horse, just continue same battle strategy as before. There are a few wall sections split up by parapets to complete here but the strategy is "rinse and repeat". On wall sections use parry and counter kills, and on parapets assassinate by dragging any enemies over the ledge where possible (if not possible then use wall section fight strategy). At the end of the wall you will have to climb scaffold to reach switch - activate - drop back down and go inside.
Don't attack monks (use eagle vision to identify threats in red). Use groups of monks to "blend" and avoid detection. Blend with group of monks that are walking in a loop at far end of corridor, and when you are behind the guard by the switch, stealth assassinate him and activate switch.
Next, another corridor with guards and monks. Have a look to get the timing of the guards and move between the monks and blend for cover when required. Blend with 1st monks and as guard passes get behind and kill with hidden blades (make sure hidden blades are selected as i failed first time as when you use switch it goes to empty hands in weapon selector). Keep moving through the monks to blend until close enough to next patrolling guard and use same method as last guard, making sure that the 2 enemies at end of corridor don't have a line of sight on you. The next bit is a little tricky with the 2 remaining guards. I used one of the bodies as bait, by dropping it halfway along corridor to entice the 2 guards to investigate, nipping into the left corridor and quickly getting behind for a double kill as they are looking at the body (again make sure hidden blades are selected as picking up corpse will de-select them). Pull switch.
Next to kill Rodrigo without being detected. Stay up high is the easiest tactic. Don't go left down the ladder; it is hard to see but there is a walkway straight ahead of you as you enter final area, cross it and jump gap, turn left and jump down to next platform. Rather than crossing rope, i opted to climb wall and edge across just to make sure i kept as much distance from guards as possible, cross to final platform and air assassinate The Pope, (or not).
I found the best tactic here was to use your "helpers" to engage Rodrigo and work your way behind him to attack, repeat until you knock him down, but it's not that easy - it never is.
Use Eagle Vision to locate the 2 switches on back wall, either side of the secret passage. Head down and battle the Pontiff for the final time; mano-a-mano.
Easiest way is get in and grab him, knock him down with 3 headbutts and once he has fallen quickly get close and grab him to drag him to his feet. Keep doing this until you get a quick-time event to strangle him.
This achievement will unlock upon assassination of Rodrigo, The Spaniard.