Buildings where the walls do not connect in a manner which is physically possible, either due to a mistake in perspective or intentionally to create an optical illusion.

See, for example, the buildings painted by "M.C. Escher":https://media.nga.gov/iiif/fa1d2a8b-50eb-4f27-834b-a3662e11e9fe__640/full/!740,560/0/default.jpg
impossible architecture