There was a deep moat outside the ancient city wall, which was once filled with water.

moat 是名词，指"护城河"，指围绕城堡或城墙挖掘的深沟，通常用于防御目的，可以灌水形成屏障。这个词源自古法语"mote"，意思是"堤坝"或"土堆"。