Crusader Graffiti in the Church of the Holy Sepulchre, Jerusalem. If it surprises you that warriors for Christ would vandalize the holiest place of the Holy Land, you should probably know that in 1099, after capturing the city and massacring most of its population, Crusaders celebrated Mass in this church still all covered in blood. Photograph by Victor Grigas. Source: Wikimedia Commons.
which reminds me
This is the better preserved of the two known runic inscriptions in Hagia Sophia. It could date to as early as the 10th century and was probably made by one of the members of the Varangian Guard, an elite unit of Byzantine army consisting primarily of Vikings (and later, Anglo-Saxons). The inscription is badly worn; the only decipherable part is Scandinavian name Halfdan. Photograph by Johnny Leo Johansen. Source: livingbycamera @ wordpress.