90 degrees. If you really need the defense, you'll build into the side of cliffs (e.g. the Anasazi and other cliff dwellers). As long as there is sufficient access to water and land for agriculture, you can pretty much build anywhere. There are terraced hillsides with angles of up to 60 degrees in various places around the world. Much above that, though, and soil isn't really thick enough for terracing. There are many villages in Europe built up the side of hills to the top because the hill makes for a natural fortification in case of attack.

The problem with things like Monument Valley-style buttes and with tepui is they tend to have little or no good soil or water on top of them (well, tepui have water from rainfall and some soil, but the tops tend to be pretty much inaccessible and not terribly suited for agriculture). If the buttes are relatively small (a couple hundred feet or so), then there really shouldn't be too much problem with habitation because that's not too far to carry water or go to tend the crops. There would likely even be steps and ladders cut into the side for access. I wouldn't expect to see much in the way of pack animals or wagons, though, without serious roads winding around the thing or a good crane system.