That zoo expands on the Hagenbeck concept – an idea Carl Hagenbeck developed in 1907, where he wanted to construct a zoo without glass or fences – and researched how far and high animals could leap, and just built moats of the appropriate size around their habitat sin his zoo.
If you live within a drive of the Washington D.C. zoo, much of the zoo is built in this way. There's even a rope over the humans that monkeys can climb across to navigate between different parts of their habitat.
However, some of the harder to contain animals still end up behind glass or bars. But then you can also get up to within an inch or two of a mature Gorilla and interact with him a bit.
I can't help but feel sad for the animals, but you can also learn tremendous amounts about them by seeing them so close.
You can read more about the original concept here: https://en.wikipedia.org/wiki/Tierpark_Hagenbeck#Panorama_ex...