The Church of Saint John the Baptist is located in the Old City of Jerusalem along Christian Quarter Road (#113). The current church was built sometime between the 8th and 11th centuries. Underneath of it is an older Byzantine Church that dates to the time of the Empress Eudocia— ca 5th century A.D.
The church houses a relic that is said to be a portion of the skull of John the Baptist. During the Crusader Period the Knights Hospitaler cared for the sick from this monastery/church. After the conquest of Jerusalem by the Muslims in 1187 — eventually the church was returned to the Greek Orthodox.
For a convenient description of this church see Bar–Am, Aviva. Beyond the Walls: Churches of Jerusalem. Jerusalem: Ahva Press, 1998, pp. 36–49.