If you're having trouble finding a treasure chest in Monteriggioni, chances are that it's in this video.
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The purpose of this video is to show you where the secret areas and chests are in the Assassin Tombs. This video has two secret chests ( to my knowledge).
The first one is at 1:15
The second one is at 3:55 ( strange way of reaching the chest, didnt know about the proper way)