moat
definition
a deep, wide ditch surrounding a castle or fortification, often filled with water
example
The castle was protected by a wide moat filled with murky water, deterring any would-be attackers.
synonyms
ditchtrenchchannel
word origin — Middle English 'mote', from Old French 'mote', 'moate', derived from Latin 'mūta', meaning 'to move, change'.