large ditch
Rare term for a large ditch or trench. More commonly expressed as 大きな溝 (おおきなみぞ) or 大溝 (おおみぞ).
On the old map, a large ditch is drawn around the castle.
Compound of 大 (おお, 'large') and 溝 (どぶ, 'ditch'). The reading どぶ for 溝 is less common than みぞ.