Please provide a reference. I'm not looking for opinions based on what or where it appears to be, nor guesses based on clues in the picture.
It's the City of Brass
The Dungeon Master's Guide (1979) says
Cover: The book cover painting shows an encounter between three adventurers and an efreet on the Elemental Plane of Fire. The fabled City of Brass can be seen floating over a flame-swept sea of oil. (2)
(Italics preserved from the original.) This blurb lurks inconspicuously at the bottom of the page after the foreword.
The title page of the Dungeon Master's Guide credits its cover to David C. Sutherland III. While this is likely his most famous piece, Sutherland is also the artist responsible for the original Ravenloft adventure module's brilliant isometric castle cartography.