Capture the Flag This Guide consists of 5 parts. One part per district + one part for the secret flags which are well hidden in the romulus lairs.
In total there are 101 flags that can be collected. This will earn you the "Capture the Flag" Trophy / Achievement.
Overview of all parts:
[Part 1] Centro District - 26 Flags
[Part 2] Campagna District - 26 Flags
[Part 3] Antico District - 26 Flags
[Part 4] Vaticano District - 5 Flags
[Part 5] Romulus Lairs - 18 Flags
.. .- -- .- .-.. .. ...- . To get this suspicious trophy or achievement you will need to solve all of the Subject 16's puzzles / glyphs. There are 10 of these puzzles in total.
This video reveals all 10 feather locations. If you collect all of them you will earn the "In Memoriam" Trophy / Achievement.
You should've completely played through the game at least once before you're doing this (just to make sure that you can enter every area). The rest should be simple then. Just go to the locations shown in the video and climb your way up to the feathers. Have fun.
Undertaker 2.0 Discover the Shrine in the Catacombe di Roma.
Gladiator Discover the Shrine in Il Colosseo.
Plumber Discover the Shrine in the Cloaca Maxima. Hard place
One-Man Wrecking Crew Discover the Shrine in Palazzo Laterano.
Amen Discover the Shrine in the Basillica di San Pietro.
Fly Like an Eagle You need to jump with the parachute off the top of the Castel Sant'Angelo to get this one. During the story you will be there for several times. In sequence 8 the waypoints will lead you your way up to the top. From there on you just need to climb this pretty tall pole that is shown during the video.
Of course you need to get parachutes first. Just do all missions for da vinci and destroy all of his war machines. As soon as you did this, he will give you the parachute as an award. Now you can buy more from a tailor at any time. Jump off the pole using + and press while in the air to release the parachute.
Serial Killer This can easily be done in DNA sequence 2, memory 6. You can replay this at any time. You have to perform an execution streak of 10 kills if you want this trophy / achievement. Do only use counter attacks and assassination kills. In case you fail, you can replay it over and over again.
Spring Cleaning I show you 3 different locations where you can find a broom. Just kill one guard with it and this trophy or achievement is yours. It doesn't matter which location of these 3 you choose, they are there at any time - no matter which ingame time it is (night / day). The people with brooms don't spawn from the beginning on, they will come in DNA sequence 7 or 8 I believe. The rest should be pretty simple then. It also seems that you need to burn a couple of borgia towers first, to make them appear. So I give you the advice to play through the game and going for other trophies first and this one much later on.
Dust to Dust This one shows you where to find an artifact. You need to leave the animus and go to the place which is shown in the video. As soon as you get at least one of the five artifacts you will get this trophy / achievement.
Keep in mind: This can't be done after DNA sequence 8 so this & "Mailer Daemon" are missable.
Mailer Daemon This can only be done until DNA sequence 8. So this one is missable (but pretty easy anyway). Just leave the animus and do what I show you to do. Then the trophy / achievement should pop.
Perfectionist For this trophy or achievement you need to get gold in 3 of the virtual training challenges. The hidden blade challenges worked very well for me, they are pretty easy with the right strategy.
The Gloves Come Off You need to win the highest bet at the Fights. To unlock this event you need to solve sequence 3 of the story, then the fights will become available automatically in the barracks. That's the place where the headquarters of the mercenaries are situated. Just go into the building, walk down the stairs and win all 5 rounds while betting 500f all over the time.
A simple trick you should make use of: Throwing sand in the beginning of the rounds with multiple enemies will help you a lot! When you are lucky, you can take out one or maybe two enemies with one hit then. Try to counter if you can and make use of a good combo with punches and kicks.