The aspect and slope of a where a vine is planted can also have a large effect on the grapes. A 30° slope facing towards the equator can double the amount of sun the grapes receive. In a flat planting, the vines on either side of any given row will block sunlight to the rows next to it. But when the vines are on a hill, the sun can hit all the grapes hanging down on every row. A sloped hill also offers better drainage, and more air circulation.
The hill also complicates the viticultural proces, because it is harder and therefore more expensive to work on a hill. It often makes mechanical harvesting impossible. In Burgundy, for example, a Grand Cru wine can be grown right next to a Cru Bourgeois, but the Grand Cru will all come from a hill, while the Cru Bourgeois will be on the flat plain right next to the hill.