When you are in Istanbul and you visit the Hagia Sofia, which is basically right next to the Sultan Ahmed Mosque, ask if you can enter the balcony inside the Hagia Sofia.
There is a carving of a Viking warrior, who fought for the Byzantinian Empire.
It basically says "I was here" and is one of the oldest proofs of mild vandalism.
A grafitti that is almoust 1.000 years old!
Hope you have a good time in Istanbul!