The mountain-side is steep, meaning it would be difficult to go up and most animals would probably not be able to do it, except maybe a monkey (because monkeys are generally considered to be agile and good at climbing), or a bird (because birds can simply fly to reach the top of the mountain). We can't really tell what the shape of the entire mountain is, but the author describes that a part of the mountain-side is flat (which means it is level), smooth (which means there are no raised areas or indentations), and steep like a wall (which means it has such a sharp incline, it's vertical like a wall).

Indeed, you needed to have a monkey It takes a monkey = you need (to have) a monkey Smooth and flat is like a mirror steep is like vertical