See the seminal discussion by Richard Krautheimer, “Introduction to an ‘Iconography of Medieval Architecture,’” Journal of the Warburg and Courtauld Institutes 5 (1942), pp. 1–33, reprinted in Krautheimer, Studies in Early Christian, Medieval, and Renaissance Art (New York: New York Univ. Press, 1969), pp. 115–150; also see Ousterhout, “Loca Sancta and the Architectural Response to Pilgrimage,” in Blessings of Pilgrimage, pp. 108–124.