This seems more like a thread for the general discussion forum, but I don't think that matters much.
European maps of cities are extremely rare prior to the 1500s (I can't speak for the rest of the world), and, as you point out, mostly isometric. I'm sure there are the occasional scattered city maps earlier than that, and probably with a high degree of abstraction. I have a feeling that maps of Chinese and Indian cities might give you some top-down (but very abstracted/simplified) examples, but can't be sure.
The earliest European city map I know of is the
Forma Urbis Romae, which was carved on marble in ancient Rome ca.203-211. Unfortunately, only fragments exist from that map, but it still gives an idea. And this is definitely a top-down/plan view of a city.
Hope that helps.
Wingshaw