fort
堡 centers on a fortified defensive structure: a fort, stronghold, or earthwork used for protection.
堡 combines 保 (protect) and 土 (earth), suggesting an earthen protective structure. The exact historical development is uncertain, but the component relationship is consistent with the meaning of a fort.
The top 保 means protect, and the bottom 土 is earth. Picture a protective earthen wall: 堡 is a fort made of rammed earth.
For ホ, imagine a fort with a single hoarse guard shouting 'Ho!' to warn of enemies: ho -> ホ.