Pour 3/4 cup sugar into a 10- to 12-inch nonstick frying pan. Shake pan often over high heat until sugar melts and turns golden, 5 to 8 minutes; watch carefully. At once, stir in nuts, then immediately pour onto a 12- to 14-inch sheet of foil. With a spoon, press mixture to flatten slightly.
Let cool, then break brittle into about 1/4- to 1/2-inch chunks. If making ahead, store airtight up to 3 days.
In a 10-inch cheesecake pan with removable rim at least 2 1/2 inches tall, mix cracker crumbs, 1/4 cup sugar, cinnamon, and butter. Press crumb mixture evenly over the bottom and 1 1/2 to 2 inches up sides of pan.
In a bowl, beat cream cheese and eggs with a mixer until smooth.
Add 3/4 cup sugar, lemon peel and juice, lime peel and juice, vanilla, and 1/4 cup liqueur; beat until smooth. Stir in 1 1/2 cups of the nut brittle.
Pour into crust-lined pan.
Bake in a 325 oven until center barely jiggles when cake is gently shaken, about 1 hour.
Meanwhile, mix sour cream with 1/4 cup sugar and 1 tablespoon liqueur. Spoon onto hot cheesecake and gently spread to cover top. Return to oven and bake until sour cream is set when gently shaken, 10 to 15 minutes longer.
Run a thin-bladed knife between cake and pan rim. Refrigerate cake until cool, at least 3 hours.
Serve, or cover and chill up to 2 days.
Remove pan rim, sprinkle remaining nut brittle onto cake, and cut cake into wedges.
