A Corbelled Lintel

Detail showing the lengths of the stone courses and a keystone apparently suspended on nothing. This last is a mystery, held up by magic. Can there be a bolt through it?

Instead of a normal arch, with the different stones pressing against each other sideways, the stones here have been corbelled. Each stone presses downwards but, because they are long and only a small part of each one projects, they cannot fall. Corbels are often used to support beams, some castles have corbelled passages, but corbelled window heads are very rare.