Preheat the oven to 350 degrees F. Butter and flour a 9-inch springform pan.
To make the cake, beat the eggs preferably in an electric mixer for 5 minutes. Add the sugar and grated citrus peel and continue to beat until the beater leaves a ribbon-like trail, about 15 minutes. Fold in the flour by hand, one-third at a time.
Pour the batter into the prepared springform pan and bake until a toothpick inserted into the center of the cake comes out clean, 25 to 30 minutes. Allow the cake to cool for about 10 minutes then unmold and cool completely.
To make the syrup: In a small bowl, combine 3/4 cup of warm water with the sugar and the liqueur. Stir the syrup until the sugar dissolves.
To form the cassata: Slice the cake into 2 (1/2-inch-thick) slices. Trim the crusts from the cake then return one of the layers to the springform pan. Spoon half the syrup over the cake then spread a layer of ricotta crema on top. Repeat, carefully placing another layer of cake, drizzling with syrup then spreading with ricotta. Wrap the cake in plastic and chill it for at least 1 hour.
To make the frosting: Sift half of the confectioners' sugar into a bowl. Add half of the lemon juice and all of the lemon extract. Stir the liquid into the sugar, breaking up any lumps. Sift the remaining sugar into the bowl and add the rest of the lemon juice. Thin the icing with a little water until it has a thin spreading consistency and forms smooth, shiny icing.
Unwrap the cassata and invert it onto a serving plate. Ice the cassata then decorate it with whole and cut pieces of candied fruit. Chill the cassata for at least 3 hours, then slice and serve.
Cook's Note: Cassata sometimes boasts a decorative layer of marzipan.