moat, fosse, trench...

outer moat (of a castle)

inner moat, moat within the castle walls

air-raid shelter, bomb shelter

remains of a dugout (usu. military)

trench, dugout

cover trench

water jump (on a racetrack, etc.), moat (around a castle

bomb shelter, dugout, trench

bunker, shelter

bunker, covered trench, dugout

trench fever

trench warfare