Successfully complete the indicated task to unlock the corresponding cheat. 
The cheats will be selectable at the "Options" menu during the replay of a memory.

Buns Of Steel: Get 100% Sync in DNA Sequence 2. This cheat makes you immune to 
all forms of damage.
Calling All Assassins: Get 100% Sync in DNA Sequence 05. This cheat gives you unlimited 
use of the Assassin bars.
Infinite Ammunition: Get 100% Sync in DNA Sequence 7.
Killing Spree: Get 100% Sync in DNA Sequence 3.
Permanent Secrecy: Get 100% Sync in DNA Sequence 6.
Ultimate Guild: Get 100% Sync in DNA Sequence 04. This cheat makes all assassins become 
Master Assassins.

Animus Data Fragment unlockables
Collect the indicated number of Animus Data Fragments to unlock the corresponding bonus:
Animus Data Fragments appear on map: Collect 50 Fragments.
Desmond Memory Sequence 1: Collect 5 Fragments.
Desmond Memory Sequence 2: Collect 10 Fragments.
Desmond Memory Sequence 3: Collect 15 Fragments.
Desmond Memory Sequence 4: Collect 20 Fragments.
Desmond Memory Sequence 5: Collect 30 Fragments.
Ishak Pasha's Memoir Pages appear on map: Collect 25 Fragments.

Challenge unlockables
Successfully complete the indicated challenges to unlock the corresponding bonus:
Altair's Sword: Successfully complete the Set 3 Assassin Challenges.
Broadsword: Successfully complete the Set 3 Mercenary Challenges.
Hired Mercenaries Stop Beggars: Successfully complete the Set 2 Mercenary Challenges.
Hired Romanies Poison Guards: Successfully complete the Set 2 Romani Challenges.
Hired Thieves Pickpocket: Successfully complete the Set 2 Thief Challenges.
Ottoman Mace: Successfully complete the Set 3 Thief Challenges.
Romani Stiletto: Successfully complete the Set 3 Romani Challenges.

Bonus costumes
Successfully complete the indicated task to unlock the corresponding costume:
Altair's Grandmaster Robes (The Old Eagle): Get 100% Sync in DNA Sequence 8.
Desmond Miles: Successfully complete all five of Desmond's Memory Sequences.
Ishak Pasha Armor: Successfully complete the Hagia Sophia challenge level.
Hagia Sofia challenge level
Collect all 10 of Ishak Pasha's Memoir Pages to unlock the Hagia Sofia challenge level.
Memoir Page locations
Search the indicated locations to find all 10 of Ishak Pasha's Memoir Pages: