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Monteriggioni artifacts
There are five artifacts to be found in Monteriggioni during Assassins Creed: Brotherhood. The achievement "Dust to Dust" requires that you find at least one of these.

Maria's Feather Chest
Upon exiting the sanctuary turn behind you and run up the wall towards the round window. Take the lift and go to the top of the roof. The feather chest is to the left (it is golden when spotted with eagle vision).
Ezio's Belt
Stick near the right wall and head along roofs towards the very front. The artifact will be on a lower level roof on a fenced balcony (it is golden when spotted with eagle vision). 
Medici Cape
Can be found on the altar inside of the church. The church can easily be spotted from most rooftops by the cross and steeple on the church.