I made a sheet of Realroad Concrete and then traced the outlines of my retaining walls to the back of the sheet.

I cut out the Concrete(s) and glued them to the cardboard retaining walls.  They went right in to place.

I cut out strips of Concrete to fit the bottom and sides of the bridge.  I glued them in place.  Any gaps I fixed with the Concrete paste.  A tiny amount will do it.

I glued the roads in place, finished a wall between the lower road and the "canal" and then glued the green down.  Notice the "jersey" style barriers on the bridge.