How are hillsides farmed? I would assume hilly land is more labor-intensive to farm than flat land, and more exhausting to humans and animals if traditional farming techniques are used. 

How do farmers plant and plow on uneven terrain? And do the furrows on a hill run uphill-downhill, or “horizontally stacked” up the hill’s incline? 

Edit: I see why sloped land is more conducive to other kinds of agriculture. Any idea how steep a grade has to be before farming grain on it is more trouble than it’s worth?
The way hillsides are farmed generally depends on the steepness of the hillside. Gentle slopes can be farmed the same way flat land is farmed.

Steeper slops require the hillside to be divided into terraces. The steeper the slope, the narrower the terraces.

Terracing of hillsides in farming reduces erosion because terraces reduce the flow rate of water down the hill. Furrows running down slope can easily transform into erosion gullies.  

Terrace farming has been all over the world and for thousands of years. It is speculated that the Hanging Gardens of Babylon, 


  may have been built on an artificial mountain with stepped terraces, such as those on a ziggurat.


Slopes in the Andes of South America were terrace mining before AD 1000.