One of those things you’ve probably seen on a hundred old buildings and never noticed. I think I recently saw – and noticed – some rather ornate wall-mounted versions in the old town of Geneva.
‘Iron containers for grease or oil to burn as lamps; usually mounted on top of a building.’
‘… an astonishing white glare from the torches, bonfires, flaming cressets, and other devices by which the river was lit up …’
Source: Virginia Woolf, Orlando, ed. Rachel Bowlby, Oxford: World’s Classics, 1992, p. 53 and note on p. 321