12 is what i know of there being tombs in revelations
You obtain Altair's armor by collecting the six seals found in the six assassin's tombs.
Florence, Tuscany, and Venice.
Yes. The Assassin Tombs are where the seals are, and the tombs are optional if you want to get Altair's armor, which is the best armor in the game.
I'm guessing you mean the 6 assassin seals for the tomb underneath Monteriggioni. There are assassin tombs hidden all over Italy: 2 in Florence, 1 in San Gimignano, 1 in Forlí and 2 in Venice.
In grand tombs.
The assassins tombs will appear as markers in your map (if you hit the back button there is a map of the whole city). just put the crosshair over an odd looking marker and it should say "assassin tomb".
they buried them in pyramids or special tombs some tombs were hidden underground.
gather the seals
There is a special armor in the game called "The Armor of Altair" It is locked behind a cage underneath the villa. To get the armor, you have to find six secret assassin tombs in hidden throughout Italy in the game and place them in the six slots where the armor is. A cutscene will appear and then you will have the armor equipped. On top of that, there are various dyes to give your original white assassin robes. Peace!
if by secret location you mean the tombs, then the answer is that if you finish all of the tombs (to finish a tomb, you have to go in one and get to the other end, where you will find a grave casket thingy, you have to open it and take the artifact that's inside it). if you collect all the artifacts, you will get Altiars Armor.
go to memory block in secret places