Over the last twenty-five years our research group, under the direction and coordination of the first author, has collected a great deal of iconographic documentation on incongruous architectural solutions, also performing some experimental studies in this regard. We started by considering the case of Medieval or Renaissance leaning buildings, such as the Leaning Tower of Pisa, the Garisenda Tower in Bologna or the typical two-storey Tilted House of Bomarzo. Several further examples and different kinds of architectural incongruity were examined, always from the psychological point of view, leading to a systematic classification and more appropriate scientific conclusions with respect to more obvious and generic treatises like the ones presented in recent volumes such as the validly illustrated ones by Jencks (1979), Papadakis (1994), Thiel-Siling (1999), Cattermole & Westwell (2007), which make distinctions on the basis of practical use, that is, residential, commercial, municipal and cultural buildings, etc.; or references to rationalisations of a philosophical kind (so-called “deconstructivism”). Our classification is based on the range of phenomenal qualities, which are well known in psychology after the contributions by Arnheim (1949), Metzger (1954, 1963), Bonaiuto (1965, 1988) and other specialists. Each quality is contradicted in each experience of architectural incongruity, with respect to traditions of regularity, order, symmetry, harmony and to preferences for right angles, vertical and horizontal planes, etc., that are aspects proposed and tacitly accepted for centuries, as dominant compositional rules: with certain exceptions becoming anomalies attracting tourist interest. Therefore, we examined cases of incongruity of position, shape, composition, size and colour. Among the psychological processes involved in the perception of these various incongruities, some particular aspects were studied: the emphatisation of the degree of conflict owing to the contrast with normal mental schemata; the attenuation of this contrast after using appropriate verbal explanations; a similar attenuation as a perceptual defence mechanism in the case of conflict overload; other processes also identified through systematic psychological research.