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 "Torre Grossa's" is located in Tuscany in San Gimignano. The tomb have two secrets areas and I show you these areas in my video.