Escher's work is often used as illustrations in science and math books, particularly those dealing with crystallography, because of his interest in and mastery at portraying the concepts of symmetry, reflection, perspective, repetition and regular partitions of the plane. But I particularly love his sense of humor. That is definitely Chartres cathedral, but he playfully switched the tall and short towers around, just to mess with our heads.

Incidentally, Chartres, France is at latitude 48.4 degrees N. so the upright depiction of the constellation Orion on the meridian in the print is inconsistent with that. Orion's belt is on the celestial Equator, and at its highest culmination would appear to be about half way up the sky at Chartres. The accurate portrayal of the constellation's star positions and brightnesses argues to me that Escher knew exactly what he was doing and bent the science deliberately to improve the composition. At any rate, he has the cathedral in this print oriented N-S, not the traditional E-W.

As a Dutchman, he would have probably been quite familiar with sailing rigs, so I wonder if the towers, the improbable sail plan, and the astronomical puns weren't all conscious little jokes he was playing on the viewer.

I have three of his prints framed in my house. This one's my favorite, for its sheer simplicity and abstraction.