If you look at the photo you'll see those plastic sheets on the right hand side. That forms the upper portion of the enclosure's wall, and beneath those sheets it is all thin wire fencing. I don't think that there's any chance of monkeys escaping, and the exhibit really is quite amazing in terms of how big it is and how lushly planted the grounds are.
Well, there's not much chance of them escaping any more. They have gotten out on a couple occasions, but the foliage has since been strategically trimmed.
