Platue
name of a polygon with 12 sides = dodecagon
Try Fjord.
Glacis - a slope, especially one that is not very long or steep
A shape with three or more sides is a polygon. Not sure there is a special name for 4 or more sides.
Mesa means a hill that has a flat top and steep sides and that is found in the southwestern U.S. Its antonym could be:canyon,ditch, orgulley
A Canyon
steep sides
A cliff
A cliff
the type of landform is a plateau.
A hill has steep sides because of how the hill formed it formed with steep sides. I don't agree. Hills have steep sides because of the Earth's plates. When the plates push against each other, they sometimes push up, creating a hill or a mountain. Whether the hill has steep sides or not depends on the factors such as weathering and erosion, and on quite simply how it forms.
"Gully" refers to a narrow valley with steep sides, often formed by running water erosion. It can also refer to a water-worn ravine or a drainage ditch along the side of a road.
On sidehill cuts or steep grades, ditch relief culverts should be installed at an angle of 60º to the centerline for more direct entrance of water into the culvert.
You might be thinking of a plateau
It is called a plateau
Yes, a valley typically has steep sides that slope down to a flat bottom where a river or stream may flow. The steep sides are usually a result of the erosion caused by the flowing water over time.