noun
dry moat
A moat without water, typically part of Japanese castle defenses.
この城には深い空堀が巡らされている。
This castle is surrounded by a deep dry moat.
水堀 is a water-filled moat, while 空堀 is a dry moat.
Compound of 空 (から, 'empty') and 堀 (ほり, 'moat, canal'), with rendaku voicing ほり → ぼり.