My M-i-L lives in the Pittsburgh area, I swear that there are no straight roads, and some of those buildings are at angles from level. The only flat places seem, like Norway, to be water.
In fact, we now joke about going straight to someplace "for Pittsburgh values of straight".
It is based on context, but there are some differences. "Bakke" can mean both flat ground and curved hills. "The ground" would be translated to "bakken" (determined and singular, or the undetermined and singular "bakke". Rarely plural). "The hillside" also means "bakke" (rely on context here) or "åsside" (lit: hill side). "The hills" translates to "bakkene" or "åsene". This can be both singular (bakke=one hill) or plural (bakker=several hills) "Bakke" can mean a steep incline in both roads and nature, whereas "ås" would only be used when describing hills in nature.