There are three ways to control draw order in TileMill. You have to keep all three in mind when developing a map because they all work together to determine how the final product is going to look.

1. If you have multiple stylesheets, they render in the order in which you put them in the TileMill project.
2. Also, just as with regular CSS, the code at the bottom renders over the code at the top of each stylesheet. In cascadenik, if you want to case a road you can put all the code into one code block because cascadenik has outline, inline, and line properties whereas CartoCSS just has the line property. To do the same thing in CartoCSS you’d use multiple symbolizers. So you’d have road_fill::outline {code}, road_fill::line {code}, and road_fill::inline {code}, in that order so that the inline styles are drawn on top of the line styles, which are drawn on top of the outline style. That way you can have your thick outline (the road “casing”), with another color on top (the road line) and another thin color inside and on top (the road centerline, perhaps).
3. Don’t forget to also order the data in the layer list at the bottom-left, which I generally keep open during development. You order the layers by moving the mouse to the left of the layer name you want to move, waiting until it turns into a four direction arrow, then dragging it where you want it. The highest layer in the list renders over the layers below it. See below: