During the story you unlock several optional tombs which are displayed on your map with a white assassins symbol in a black circle. For each tomb you will obtain an achievement and a seal. With these seals you can unlock Altair's armor in Monteriggioni.
The Assassin Tomb called "Il Duomo's Secret" is located in Florence in the San Giovanni District. The tomb have two secrets areas and I show you these areas in my video too.