The boulders on the outside of the wall would be quite large, so they couldn't be pulled down easily. Stones about a foot and half in diameter weighing over 100 lbs. Stones on the inside wall about the size of honey dew melons, (six or eight inches in diameter). People have been building field breastworks this way since Roman times. Not all fortifications were triangle shaped. It's just easier to defend three walls than four. Smaller positions were built in a circlular pattern, which may or may not have had several shooting ports added to the design. Such structures were primitive, temporary, defensive positions and were not intended as a permanent shelter. You won't find evidence of roofs or flooring. Nor is it unusual to find cannon ordinance buried in these structures, because the indians couldn't forge iron. So there was no danger leaving it behind. A retreating force often did abandon their heavy ordinance in order to move faster.