Inner Moat
noun
The inside ditch is the inner ditch, or, if you make it awesome enough, the inner moat. If they get through the outer moat, we have the inner one, at least!
うちぼり
The reading is the kun'yomi / vocab readings of these kanji. 内 + 堀 added into one word.
私は彼を内堀に通すために道をあけた。
I stepped aside to let him pass through to the inner moat.