Follow the water. Where does it go ? This waterfall recycles the water after driving the water wheel. If you look carefully you see that the construction is made of three impossible triangles.
The building seems to be solid, but when you examine it you see that the monks are ascending or descending forever. Escher didn't invent the endless staircase but he managed to draw it in a very appealing way. The inventors of the endless stairs were L.S. and Roger Penrose in 1958.
The impossible stairs reduced to its simplest form. Even now it's hard to see what's impossible about it. Father and son L.S. and Roger Penrose were the first to publish its existence in 1958.
It's really incredible, but these guys have succeeded in building Eschers Ascending and Descending in Lego. You can read on their site how they've done it.
Escher had a lot of fun with the perspective here. He also invented the little centipede creatures, called "Wentelteefjes" in Dutch (a hard to translate pun). He even described how they curled themselves up and rolled along.