This is a collaboration between my self and Peter Raedschelders. [link]
The city has 180 degree symmetry, flip it over and it's the same. You can see a woman on a balcony being serenaded by her admirer on both sides. Boys are using this strange symmetry to play an odd game of catch. The tunnel in the city gives conflicting signals: It looks like it goes straight up but it connects the forward corner to the back corner. This impossible figure is based on two joined Penrose triangles [link]
. Here is a Wikipedia article on the Penrose Triangle: [link]
I didn't see this strange tunnel the first several times I looked at Raedschelders' print. To call attention to the tunnel I employ angel and devil tessellations (of my own invention). The angel tiles gradually become 3-D, rise through the tunnel then reunite with their sisters. The devils also leave their 2-D world to descend through the tunnel and rejoin their brothers.