Preheat the oven to 400, and lightly spray a 8-inch square pan.
In a large bowl, combine the sliced rhubarb, flour, sugar, rose water, allspice, and orange zest. Stir this mixture together very well.
Pour this mixture into the prepared baking dish, and set aside.
In the same bowl that’s now empty, make the topping: add the flour, brown sugar, oats, allspice, and salt. Stir very well.
Pour the melted butter on top, and stir to make a crumble topping. You might have to clump it together with your hands a bit.
Scatter the topping evenly over the rhubarb, and slide it in the oven for 20-25 minutes, until golden brown and bubbling around the edges.