Preheat oven to 350 degrees F (175 degrees C). Grease a 10-inch fluted tube pan (such as a Bundt(R)).
Sift flour, salt, baking soda, cinnamon, nutmeg, and allspice together in a bowl. Cream butter and vegetable shortening together in a separate large bowl until thoroughly combined; beat in white sugar and 1 cup brown sugar until the mixture is smooth and creamy. Beat eggs, water, and vanilla extract into the shortening mixture, followed by the flour mixture. Gently stir in zucchini and 1/2 cup walnuts. If batter seems dry, mix in 1 more tablespoon water.
Melt 1/4 cup butter in a saucepan over medium-low heat and stir in 1/2 cup brown sugar until it has dissolved and small bubbles appear in the mixture. Stir 1/2 cup walnuts into the brown sugar mixture.
Spoon brown sugar-walnut mixture into the bottom of the prepared cake pan and pour cake batter over the mixture.
Bake in the preheated oven until lightly golden brown and a toothpick inserted into the center of the cake comes out clean, about 1 hour. Let cool in the pan for 10 minutes, then invert over a serving plate to reveal the brown sugar topping.
