dry moat
隍 centers on a dry moat: a defensive trench around a castle or fortification that is not filled with water.
隍 combines ⻖ (mound, hill, or earthwork) with 皇, which likely contributes the sound. Together they suggest an earthen defensive structure, specifically a dry moat.
The left side ⻖ represents an earthen mound or wall, and the right side 皇 suggests something grand or imperial. Picture a grand castle with a dry moat dug into the earth beside its walls.
For コウ, imagine a dry moat around a grand palace, and think of the emperor's court (皇居, こうきょ) where such a moat might be found: コウ from 皇居 cues the reading.