To unlock ALL the abilities, you would need to reach level 50 - each level up you get unlocks something new for you, although not necessarily an ability.
You play the game!
u cant only in multiplayer
find 6 keys of romulous
There are four cahracters for you to unlock but you start with ten default characters.
if you mean in brotherhood u unlock it by destroying all of Leonardo's war machines and going to one of his benches
More Lairs will become available as you progress in the campaign.
You must get 100% sync in DNA sequence 6.
u cannot get the medici cape in AC brotherhood u get it in AC 2 u get it after u killed the pazzi family
You cannot unlock the costume in game. The only way to unlock the costume is by spending Uplay points through the in-game interface.
The Highest Level you can get to is Level 50
You just have to keep leveling up or getting achievements. Hope this helps!
The only official "secret areas" are the Lairs of Romulus, which you can complete to unlock the Armour of Brutus and the Dagger of Brutus.