Quito: on flat land surrounded by hills, not in mountains.
Bergen: on a lake surrounded by hills, not mountains.
Trondheim; built on flat/rollnig land, not steep mountains.
http://trendsetter-graz.fgm.at/pics/stadtbild_graz.jpg : built on flat land by a river, not mountains.
Tblisi built on flat land by a river, surrounded by hills, not mountains.
http://www.old-picture.com/europe/pictures/Constantine-General.jpg Hills, not mountains.
http://www.photoglobe.info/bl_guatem...emala_2144.jpg Built on flat/rolling land.
Bogota: built on flat land surrounded by hills.
You can already build cities on Hill tiles in Civ - and this gives a large natural defensive bonus.
Hills are not mountains. Civ mountains are like alps and himalayas; very steep, mostly above the tree line, snow-capped, impassible.
You do not see any cities high up in the alps, or himalayas, or Rockies. Incas are pretty much it for building a few sizeable cities on incredibly steep terrain. There are some people living in very steep terrain in places like Nepal and Bhutan and Afghanistan, but not cities.