If you want your gradation to work, the items need to be similar and progressively stronger. That's why it doesn't work. A wall is not "stronger" than a moat. "Not a fence, a rampart" would work.

Compare to the canonical example from Cyrano de Bergerac: ''Tis a rock! ... a peak! ... a cape! -- A cape, forsooth! 'Tis a peninsular!'