Line a baking pan with parchment paper. Preheat the oven to 350 degrees.
To make the base, mix the flour, baking powder, sugar, and salt. Stir.
Add the water and oil. Stir together to form a crumbly mixture.
Press the mixture into the base of the pan, then pat it down to an even layer.
To make the crumble, mix the brown sugar, flour, salt, and cinnamon.
Add the water and oil, and stir until you get small crumbles that are still a little powdery.
Toss the mixture a bit using your fingers until you get larger crumbs. They shouldn't be any larger than a pea.
To make the filling, peel and slice the apples. Slice them as thin and uniform as possible.
Toss the sliced apples in the lemon juice and cinnamon.
To assemble, pour the coated apples onto the base in an even layer without gaps.
Drizzle 1/4 cup of the caramel sauce over the apples.
Sprinkle the crumble mixture evenly over the caramel and apples.
Put the pan in the oven and bake for 30-35 minutes or until the crumble on top is browned.
Let the bars cool in the pan before removing and cutting. Drizzle the remaining caramel sauce on top before serving.