Walls have a very important place in Mexican life, and I think, from what I have gathered, that they always have. The enclosed courtyard was a feature of Spanish (and before that, Moorish) architecture and was handed down to colonial Mexico, of course, but I am looking into the meaning and importance of walls and private spaces in pre-hispanic life as well.
On a purely aesthetic note, I find the walls of Oaxaca to be extremely beautiful, whether carefully catalogued and tended or gorgeously neglected. I especially like to take pictures of saints and icons set into the walls of public buildings:
San Fransisco, I think
Jesus, in a gorgeous royal blue niche.
I love the archeological layers revealed by an ancient wall's weathering... This wall has lost it's stucco and reveals brick and stone underpinning. It is a parking garage.
Homero in front of a cliff wall painted with petrogylphs near his home village of Santa Maria Intundujia.
A catalogued wall downtown, losing its stucco.
A wall made of cantera, with bricked up windows.
A close up of an interior wall in Mitla, prehispanic site near Oaxaca. This was a priestly habitation, and the walls are of mosaic.