The Wall: Bake a cake in a 8" square pan, cut it in half and stack halves together with filling for wall shape. Ice thickly with pale brown buttercream and mark off "bricks" with toothpick.
Humpty: Bake two half egg cakes in an egg-shaped pan. Chill and trim halves flat with sharp knife. Ice the flat sides and push part of taffy apple sticks into one half, then gently press halves together, pushing second half onto stick as you do. Cut off a slice of resulting "egg" so Humpty can "sit." Push a taffy apple stick up into each half of "egg" then very gently push egg and 2 sticks down onto wall.
Ice Humpty's body half flesh color, half pale green. Ice rolled-out marshmallows for ears and a miniature one for nose, attaching all with toothpicks. Figure pipe arms and legs with tip 2A, hands with tip 4. Do shoes ahead with tip 8, using royal icing so they stand out stiff. Use tip 2A again for cheeks, eyes and pupils, piping these right on cake and flattening with a fingertip. Use 2A also for buttons. Do collar and bow with tip 104. Arrange pink drop flowers made ahead with tip 190 and 191 at bottom of wall, pipe leaves between with tip 67. Add top hat!