Roman Arcade with Engaged Columns
An arcade is a passage or walkway covered over by a succession of arches or vaults supported by columns. An engaged column is a column embedded in a wall and partly projecting from the surface of the wall, sometimes defined as semi or three-quarter detached. In Roman architecture they exist in profusion, most commonly embedded in the cella walls of pseudoperipteral buildings. Engaged columns are distinct from pilasters, which by definition are ornamental and not structural.