When the Knights of Malta were building the fortified city of Valetta in the late 1500's, they dug a deep ditch on the west side of the city to stop attacks from land. The other three sides were surrounded by water and tall rock cliffs with built up walls on top. In this photo, taken from the bottom of the "ditch" one can see "modern" bridges that span the ditch for traffic to get into the city. This photo was taken in March of 2007.
See below for more information on Valetta.