protection
戍 centers on guarding or defending a place, especially a border or frontier. The single given meaning 'protection' captures this idea of standing watch to keep something safe.
戍 combines 戊 (a halberd-like weapon) with 丶 (a person or mark), suggesting a person with a weapon standing guard. This is a standard phono-semantic explanation.
The outer shape 戊 looks like a halberd or weapon, and the inner dot 丶 is a person standing watch. Together they show a guard protecting a border.
For ジュ, imagine a guard protecting a jewel: jewel -> ジュ, and the guard stands with a weapon to keep it safe.