Porthole?

It's amazing what odd little things one notices while stopped in traffic. 
This little window is sited at the top of a flight of steps leading to nos.  116 and 118 Hanover Street, twin main doors of a New Town tenement, a railing divides the steps and runs right up to the window. It has been altered to receive window bars but is shared  between the two properties, divided on the inside.

Symmetry being a notable feature of the New Town, the property directly opposite bears the same design but the original, round shape, unaltered except that the railing has been removed.